Due to the previous issue with the incorrect second cycle combiner, the devs added a 1-pixel offset to texture coordinates in this texrect to compensate for the combiner error.
In 1-cycle mode, the two combiner cycles should be identical. Using Texel1 here in the second cycle, which is the actual cycle of the combiner used on hardware in 1-cycle mode, actually samples the next pixel's texel value instead of the current pixel's. This results in a one-pixel offset.