I believe that merely points out the level of precision to which those bikes were made. Are you going to find hand-made bikes (especially those made during the frantic boom years) a little "off" here and there? I believe so. Take into account the level of accuracy of the instrument and you are within a half a degree of the spec on both angles. I measured multiple FB frames with a similar gauge and they all came out to within a half a degree (or less) of 73 degrees parallel on the HT/ST. Schwinn sourced tubular forks from multiple suppliers, tolerances and variations thereof may also throw off the TT angle. I'm convinced it was intended to be horizontal, just as the HT/ST angles were intended to be 73 degrees. The reality varied somewhat.