Most rental companies will only guarantee a specific class of vehicle, not a specific make or model. It's all on a first come, first serve basis. If they happen to have a Mustang on the lot that day, that's what you get. If not, TS.
Sometimes what they lump into a class defies logic as well, and even varies by region to region depending on whether stores are franchised or corporate owned. I found that out on my trip to Halifax when I booked what I thought was a Fiesta or Corolla, and got an old ass second gen Kia Rio. I chirped them good for that.
Another thing too. If you get a stupid cheap rate, and they have multiple makes of vehicle in that class on their lot, they're always going to give you the crappiest one. Hotels do the same thing with rooms booked online at deep discounts. The travel industry does not reward penny pinchers.
Getting the class down then negotiating an upgrade is a good trick. Just keep in mind that a lot of places limit their counter staff fixed rates to force people to go through the call centre. At least that's how things worked where I was.