Judging from the narrative of OP, he is probably talking about the free spin from the bonus round that usually get if you hit certain number of scatter on a regular bet.
It usually have higher chance of returning big profit since the profit mechanism on this bonus round has special effect that boost winning percentage on each spin.
The free spin given by the casino which is just a regular spin is just a normal spin that is literally free. I do get a lot huge win too on that kind of free spin by hutting bonus round.
I never really understand how these free spins can be different entirely aside from the means through which they are awarded and some game specificity to its usage. Having to read all these promotes the idea of, some free spins are a lot fair than others don’t you think?
The game is supposed to be fair and not have high probability of success due to the means certain bonuses are being accredited.