Ok so i have a GT bike (aggressor 2), I crashed and I went to my independent bike shop to get it checked out,
He said the gears need adjusting which was about right, and he also said that the cassette and the rear dérailleur were not compatible because they are two different makes and he explained that the distance between each gear on a particular make would be different from another so he could keep on adjusting forever and ever but it would never be perfect

i had a sram 9 speed cassette with a shimano deore rear derailleur

now both are shinamo

did i just get tricked into buying a new cassette?
or was he telling the truth please let me know