No way the Russians are totally pulling out of Syria - they went to war to preserve their access to their naval base at Tartus. I’d bet they plan to keep their new air base too.
Some specialized units (like artillery) won’t be needed when combat ops wind down, so naturally some troops will head home.
The announcement of withdrawal is for public relations, and to try to pressure/manipulate the US into a foolish withdrawal (while Iran sends their surrogates in).
Nauert: If Russia chooses to pull out, certainly, that is its choice to do so, but we continue to work through all our partners to try to stabilize the country.”