In this paper, we study how mobility affects mobile data accounting, which records the usage volume for each roaming user. We find out that, current 2G/3G/4G systems have well-tested mobility support solutions and generally work well. However, under certain biased, less common yet possible scenarios, accounting gap between the operator's log and the user's observation indeed exists. The gap can be as large as 69.6% in our road tests. We further discover that the root causes are diversified. In addition to the no-signal case reported in the prior work , they also include handoffs, as well as insufficient coverage of hybrid 2G/3G/4G systems. Inter-system handoffs (that migrate user devices between radio access technologies of 2G, 3G, and 4G) may incur non-negligible accounting discrepancy.