I'm not sure I would say it "varies a lot". Flying westbound is a fairly recent development in so far as it's been several times now over the prior month. If you look at SQ23 YTD, over 90% of the time it goes Eastbound as northern hemisphere winds prevail West to East. In the northern summer, winds over the Pacific are weaker so the option will be available at times. However, generally, the winds are quite strong over the northern Pacific which would preclude flying in that direction if other options exist. Yes, pre-Ukraine war, the polar route was fairly common ex-JFK but extremely rare ex-SIN.

Reminds me of my CX LAX-HKG flights...there were days in the winter when the 744 had to refuel in TPE. Just couldn't make it when winds were particularly strong.