As others have said, mushroom compost varies from place to place - the nearest one to here comes from a mushroom operation that uses chicken manure-and-something (sawdust, when I was watching) which is first composted (they use some big machinery), sterilized with steam, inoculated with Mushroom spores, and when they're done and harvested, the stuff is bagged up and sold.

All "mushroom" compost starts out as animal manure and becomes "mushroom" compost after mushrooms are grown in it.

What a great question -- you made me look it up.
I read somewhere (not sure if true) that mushroom compost is usually basic (and for that reason, not to be used on my blueberries). That would be one difference.

Mushroom compost is a by-product of mushroom farming.

I think it does not have as much nitrogen as manure compost (since the mushrooms used most of it already) and there's a warning that the salts in pure mushroom compost may burn seedlings. Depending on the supplier, it may also have pesticide residues?!

Ok, now I'm wondering WHY I put 2 bags of it on the veggie bed instead of using cheaper chicken manure compost. Phooey!
:(

These dudes oughta know about mushroom compost.

Here is a link that might be useful: American Mushroom Institute (AMI)

Don't take this as the gospel truth but I was always told you had to watch mushroom compost if you are using it for food because often the mushroom farms used horse manure which has the potential to contain many more residual medications than cow manure because what you are allowed to put in a cow is regulated because it is considered food when a horse is not.

FWIW.