The quote from the novel seems to offer a couple of alternatives, but nothing genuinely definitive. Freya posits that it was to make the returners feel lighter. Badim argues that it might be because the ship's computers lacked calibration after being reprogrammed during the '68 revolt.
They are in 1g, by definition, but the voyagers decide, and the records in the computers they brought down with them confirm, that they were living in something close to 1.1g for most of their voyage home. Why the ship did this, they cannot determine from the records they have.
Freya says to Badim, “It must have done it to make sure we felt light when we got here.”
“Yes, I guess that’s possible. I suppose. But I wonder too if there was some programming done by the people in Year 68, some kind of alteration that left the ship with no frame of reference. We can ask it when it comes around the sun.”
There doesn't seem to be a resolution why this occurred, except perhaps the subtle implication that it may have simply been a programming test that never got reversed or that it had something to do with the "enforced forgetting" regime.