When I add the numbers given of the sons of Kohathites + Gershan's + Merari's it totals 8, 600. But the sum given in verse 48 is 8,580. What am I not counting correctly?

Try recalculating;
Kohathites = 2750 (verse 36)
Gershonites = 2630 (verse 40)
Merari = 3200 (verse 44)
which when you add these you total 8580