Could be a few things...
It's possible that when the creditor updated the closed account to a $0 balance, that they also updated the payment history for an additional month -- was this a charge-off? Did they update additional months with a 'failure to pay' / 'CO' payment status? Did they update the 'Time since late' field? If so, the damage of the negative updating may have outweighed the benfit of reducing the util.
It's also possible that you have been assigned to a new FICO scorecard since you had the last collection removed and are now being compared to others that have no collections on their reports. When this happens, it is possible to lose a few points because, even though you've been moved to a cleaner scorecard, your overall profile may not be as good as others (for various reasons - age, util, number /type of accounts, etc.) within the same scorecard. So it's kind of like you were at the middle / upper echelons on your previous scorecard and are now in the lower / middle echelons on your new scorecard. The good thing is that with moving to a cleaner scorcard comes a higher ceiling for score growth over time.
