From my experimentation, I would assume its a cap on the stat bonus, in this case total hp.

If two HP+60s are melded, they would have a chance of somewhere near 16.5%

If two HP+40s are melded, they would have a chance of somewhere near 25%.

It doesnt affect your percentage if they are a high T3 or a low T4, its the bonus to stats that computes the chance of success.