Question about salary compensation?

How is this allowed at my job? I work at a pretty large company and I was promoted about a year ago. I went from hourly pay to "salary" pay, at least that's what I was told. The weird thing I find about it though is my pay is always fluctuating, its never a set amount like salary is supposed to be. It's always in a range of $100 or so. My boss treats me like I work hourly, he tells me I need to get my 40 hours in the week, and if I dont I have to work Saturday usually for a few hours to make up my time. My job is fairly unique since I pretty much leave when the job is completed and since I'm a pretty efficient employee, I can usually get my work done sooner than what the schedule says. Another thing my boss has told me is that if I dont hit my 40 hours, he will take that time away from my paid time off. I understand as a company they want me to work 40 hours to get my "set" rate or whatever, but my biggest issue is what I've said above plus my supervisor doesnt schedule me remotely enough for me to get my 40 hours on a weekly basis.

I always assumed that if you are salary, you get paid the same amount if you work 50 hours, or if you work 20 hours. And with my job, sometimes it gets really slow so I could probably, in certain months in the year, work for about 25- 30 hours in the week. Another bit of important information might be that I also get paid overtime if I go over my 40 hours, so maybe that's how they justify it? That would make sense, but they are completely apposed to overtime, I've only gotten overtime a few times this past year. I've tried looking up the legal definition of salary and it seemed like it was what I said about working any amount of hours you get paid the same amount, so I figured I'd ask you guys about this to see if anyone knows.

If you'd like more info, let me know, I really want to know if this is common and I just dont understand salary or if this is a rare thing companies do, or if this isnt allowed. Thanks!