I suspect two factors - as you mention, in winter the jet stream is stronger so westbound flights are likely to take longer (but conversely, eastbound should be quicker).

The other factor is that the IAH flights tend be routed over Newfoundland and down the Eastern slight of the US - which is where the jet stream sits during the wintertime; SFO/PHX flights tend to overfly Greenland and Hudson Bay and so tend to be less affected by the jet stream.

On a side, I remember extremely fact BOS-LHR and ATL-LHR flights in the past with a tail wind of 100+mph which seriously shortens the trip. Conversely, I have had very slow, tiring westbound flights with not dissimilar headwinds with a LHR-IAD flight taking more than 10 hours! I had a meeting in Geneva a few years ago where many of the participant's westbound (from Asia) flights had been diverted due to low fuel caused by strong headwinds!