I don't think they take bikes off the road during the winter, or at least, they don't take enough off the road to be noticeable relative to the decreased number of trips that other people are taking. Winter is prime commuting for me because I can always get a bike at the nearest stations. The stations are undersized for peak summer commuting traffic, but fine for winter.
The chart says "Unique Bikes Used Per Day". Q1 2015 was a very cold winter, and I can imagine the system being so under-utilized that there were many bikes that sat unused through an entire day.
