The PP System is weighted. If you look at your top ranks you can see all of the weighted values listed below their raw values.
Currently 76pp for you is weighted at 10%, so you get around 8pp for that play, but all of your other top plays worth less than 76pp are weighted less, which detracts from that 8pp. In this case you lost 7pp from your other scores being weighted less, resulting in a net gain of 1.