What epoch are we in?

I am doing some workflow experiments using various virtual observatory tools. In one example I downloaded the USNO-B and 2MASS catalogs for a 1 degree region around the young star cluster in NGC 2264.

With TopCat I did a cross-match, and extracted a few interesting quantities and plots. The first plot is the total proper motion vector from USNO-B vs great circle distance between the USNO-B and 2MASS positions on the sky. The color weighting corresponds to the angular separation divided by the total proper motion vector or the net difference in epoch between the two datasets.

Now I just plot the net positional difference versus proper motion in right ascension...

... and declination. Err, both are quoted as epoch 2000, so what is going on here? Naively it seems the epoch of the two datasets are separated by 6-7 years. I know the 2MASS are 1999.9 but what about those USNO-B positions...