It's kinda hard to tell from just that.

Companies usually split the miles into two rates. Say, if you drive 2000 miles @ .34cpm. It will pay something like 2000 miles @ .25cpm (the taxed amount) and 2000 miles @ .07cpm (the per diem amount). The company usually keeps a couple cents so you do end up getting paid less per mile.

What you are seeing there is an accounting thing. Everything has to zero out in the end. Looks like you were paid $318.12 for your per diem in March.

That looks more like a weekly per diem. Like vinsanity said they play with the numbers for bookkeeping. You should have a pay sheet with a break down of your trips and then the actual pay stub. If you compare the two, then you can figure out what they done. Looking at one or the other you can't see the whole picture.

You are better off opting out because it makes your yearly W-2 look like you only made half which robs you, reducing many benefits. Come tax time it all comes out the same anyways. Companies found a way to save thousands with this per diem option by reducing what they have to match with social security, workers comp insurance and 401K to name a few.