explain to me how the illogical premise works?

But that's kinda the point. You really don't know what you will be facing. That's why they never anticipated the kett and the other races I can't even remember their name so worthless they are, as well as their actions. In 600 years, even being a little in cosmic time, a lot of shit can happen, especially if you don't know anything about the races that inhabit that galaxy, and at best you know planets' climates. It might not, but they shouldn't really be surprised that things didn't go as expected. 

You really can't know if the probability is low or not if you don't know the variables. I mean, if you only know about the planets' climates and you don't know anything about the races that inhabit that galaxy there is absolutely no way to know what is going on there. 

I am with alanc9 in so far as one could probably calculate the chance that any unexplored cluster in the Milky Way would be colonized by any species within 600 years. Given that all races in the MW combined only explored (not even colonized) under 1% of the galaxy, that number would likely be extremely low. And that's with mass relays and a whole bunch of sophisticated paleotechnology already present.

So I can see how it can make sense to have a predictive model, that would estimate an extremely low chance for another species to migrate into Helios within 600 years. So far so good.

However, that would imply that np species is there already and that the MW species knew this from their scans. But in 2186, before the scourge and the kett, the angara were reportedly a thriving type II civilization, settling on about a dozen worlds or so. Hell, their home world Hawarl was even scanned and evaluated and apparently, no evidence of the angara being there was ever reported. So either their geth-relay-superscanner-thingy was complete shit and they just relied on it blind, which would be stupid or they were aware that their scanners couldn't resolve the question of whether or not any civilization was already present on any of these worlds (in which case they really shouldn't have already divided up worlds that might already be owned by someone). The third option is that the Benefactor didn't give a dam and fully expected the Initiative be an invasion force but there is no indication of that in the game.

In any case, the fact that the angara were already there and thriving by the time the AI started makes Helios a really bad choice for an initial landing zone in Andromeda, even if the Initiative didn't outright know about them when they left.

I'm not talking about migration, I'm talking about the races/machines that ALREADY lived there, like andromedary said.

So, their technology didn't warn them about the angara? The kett? The remnant? Any kinds of synthetics? 

Either you're using a bad technology, or you're taking hell of a bet. 

Well, then the whole Andromeda plot is based on something that had a very low probability of happening, according to alanc9. And even more unlikely, since not only one, but two races appeared. 
It seems the probability isn't so low after all! 

RPG plots are often based on things with a very low probability, unless we're doing some sort of Chosen One thing, or there's something like the Force screwing with everybody's destinies. If Saren and Sovereign hadn't made such a botch of their attempt to activate the Citadel relay, Shepard never gets hit by the beacon and there's no stopping Saren. Actually, ME1 has a bunch of them -- what are the chances that an incriminating conversation of Saren's would somehow find it's way into Tali's hands? DAI is literally an accident which happens to the PC.

To Start welcome back Dutch  But if you played the game they left because of the impending Reaper attack (Hint Benefactor) where they got the tech.? well Jene (think that's spelled right) was a very wealthy woman. so use your immageition and again the benefactor  and no it has nothing to do with reaper tech. other wise they would be indoctrinated. but Nice try keep up the well work????

Familiar to who, exactly?

Edit: I mean ,  they could have thrown a planet in there which neither kett, nor exiles, nor angara had ever set foot on, and when we got there we'd do.... what?

Ryder's the first MW person to set foot on Aya, and was only beaten to Voeld by the dead crew of the Periphona and to Havarl by a handful of crashed turians.