synthetic population growing on some planets ???

I agree, I don't think it's supposed to do that.

I've been playing as a custom civ (synthetic) and can confirm that the pop is growing on some planets. It must be a pretty low value though, as planets with a population of 1 or 2 don't grow.

I've played through 7 games as YOR and have not seen this.

Can you provide additional info about which planets are growing? Were they colonized by you or taken over from other empires?

they were all ones i colonized.  I first noticed it with a planet i had settled with 2 pop.  When i went to shuttle over another 2 (several turns later) it had a pop of 5.  Out of 12 planets 3 were growing their own population at a higher than normal rate (don't recall exactly what it was but it was ~1.0 every 4 or 5 turns)

I must still have lots to learn about this game even after 100 hrs+...even if I am now on my 1st play through of a synthetic race.... OK Synthetic races don't grow??? How then is population increased? I know they need durantium and if they start at 2 on a 4 planet I assumed they grew to four slowly and then I build a city and that gets them going?? Also how do you shuttle civs to another planet?

There's a difference between synthetic and silicon based.  Silicon (Slyne, Onyx) grow at the same rate as carbon based, but require durantium instead of food to build cities.  Synthetics like the Yor aren't supposed to grow by themselves, there are colony projects you use to increase the population (i.e. your robots spend time building more robots).  Synthetics don't have population caps, so they don't need cities.

And it's helpful to keep a leftover colony ship to shuttle robots from your homeworld to new colonies. (Your HW will probably end up being a major production center for the first half of the game and critical to robot success)

Never played Robotic civ....so got that wrong.

Give it a try!  It's not my favorite, but some folks really like it.