Remember that you get the 10 MP permanently when you get the Strong Swings enhancement - whether you use any weapon or not. So when you use sickle you will not get any extra compared with bare hands - the bonus MP is already included. When you are in 2-handed style, you just get 15 extra.
For me it works like this:
No Strong swings, no weapon or weapon that does not use THF style -> 69 MP
No strong swings, weapon that uses THF style (dwarven waraxe, greatexe etc) -> 75 MP (due to THF feats total 6 MP bonus)
Strong swings, no weapon or weapon that does not use THF style -> 79 MP (10 MP bonus from strong swings)
Strong swings, weapon that uses THF style -> 100 MP (6 MP bonus from THF feats, 10 MP bonus from strong swings, 15 bonus due to using THF weapon while having strong swings 69+6+10+15=100)
So, MP displayed in character sheet matches expected values with strong swings. Works fine IMO.