Question about TTX

I've tried running a few fonts through TTX and back and have noticed a few puzzling changes:

(1)  All the glyph bounding boxes get recalculated, often to 15% to 25% smaller than original.

(2) The numberOfHMetrics value in one font got changed from 141 to 140.

The questions are:

(1) Can shrinking the bounding boxes cause any compatibility problems with old apps that may have been fine-tuned to use the older, sloppier boxes?

(2) Any idea why TTX would compute a different number of Glyphs than in the original TT font file?

Any answers appreciated.

Regards,

George