WW:

I agree with Oren on everything except the terminology. Oren says that different toilets come with different "offsets", and I call those different "rough-ins".

The rough-in is the distance from the middle of the toilet's drain pipe to the surface of the finished wall. Nowadays, most toilets are made to fit a 12 inch rough-in. However, most toilet companies will make toilets that fit a 10 inch rough-in and some will make toilets that fit a 14 inch rough-in. In your case, you probably removed a toilet that had a 10 inch rough-in, and you replaced it with one with a 12 inch rough-in, and now you're discovering that it won't fit properly. If you return the new toilet you have for a refund, and buy a toilet with a 10 inch rough-in, you should be OK.

As I did a Google search for "standard rough-in", I came across this page on e-Bay listing a fair number of brand new toilets that are made with a 10 inch rough-in:

10" Rough in Toilet | eBay

Also, now that you have the toilet out, it's a good idea to paint the area directly behind where the toilet tank will be with several coats of interior oil based primer, which you still should be able to buy, followed by a couple of coats of Zinsser's Perma-White Bathroom Paint. The primer will be impervious to moisture, and you need that in the small gap between the toilet tank and the wall behind. Without that oil based primer, any condensation that accumulates on the wall behind the toilet tank will seep into the plaster or drywall and cause water damage to the wall there. Painting with an alkyd primer prevents the moisture from being absorbed into the plaster or drywall, and thereby prevents that water damage.