Is this right?

So my husband been working at 7 eleven for a year or so and got promoted to assistant manager at the beginning of this year and is being paid 11.50 but recently looking at his paystub he notice and he gets paid weekly the first week they pay him 11.00 hr second week 10.50 hr and the last time he got paid it was 11.50. Can employer do that keep changing the pay on him like that. And that was month of March but in Jan and feb is getting paid 11.50 why all of sudden they change how much he gets hourly he works 2-10 five days a week is that shady or can they do that with out him knowing it.