In the Character Sheet under the Combat Abilities the tooltip for Summoning is listed as a 5% improvement per point but is actually 10%. Almost everywhere else lists it properly as 10%, but this particular tooltip is incorrect.
On a related note, I feel like 10% per point is more-appropriate than 5%. Since the benefit only applies to Summons, and there are no other stats to increase their power, 5% would be too small. So fix the text, not the function. :P