Per diem

Hello fellow professional drivers. 

I have a question about per diem pay. I’m have been working as a hot shot driver for a small privately owned company in Alabama. When I signed on I was told I would be paid 20% of all loads taken. I have been going over my pay stubs (I am a W2 driver) and I’ve noticed something. I’ve noticed that I’m not being paid but 15% of the load and then 5% or there about in per diem. Example my last check should have been for about 1230 before taxes for three loads hauled but on my pay stub it shows like 923 then like 295 under per diem pay. I was only away from the house for  three nights and if I’m figuring this right at the IRS rate of 66 bucks a day for three nights away from home doesn’t add up to 295. 

I may be wrong ( and I probably am) I thought per diem was something totally separate of load pay. 

Is something fishy or is this how things are supposed to happen? 

Thanks for any advic