That is because your other scores weight less.
Let's imagine that your top rank only has 2 maps and looks like this:
#1. 100pp, weighted 100% <--- worth 100pp
#2. 100pp, weighted 95% <--- worth 95pp
Your total pp will then be 100 + 95 = 195pp. If you now set a 110pp score, your top rank will look like this:
#1. 110pp, weighted 100% <--- worth 110pp
#2. 100pp, weighted 95% <--- because of your new score, this score is pushed down, making it worth less. (now it's 95pp instead of 100pp)
#2. 100pp, weighted 90% <--- same as above
As you can see, your new total pp is not 195 + 110 = 315pp, but 110 + 95 + 90 = 295pp.