I'm no expert, but here's my guess.
When Dell was showing late with a balance, it factored into your utilization. For example, let's say Dell was reporting a balance of $1,000, and your only other tradeline was a credit card with a $1,000 limit and a $90 balance. Your utilization would be 109%, since you owed $1,090 on a total open credit limit of $1,000.
When Dell charged off the account, they then report a $0 balance. Your utilization drops to 9%, since you have a $90 balance on a $1,000 credit limit.
The 120 days was already a pretty big derogatory, so the hit from going to charge off was smaller than the gain from the lower utilization.