The most likely cause is that iChart software is busy with a synchronization command at that time. iChart has a function to schedule a synchronization of the iSIC real-time clock (RTC) to the PC clock at the beginning of every hour. As soon as an hour begins, iChart sends the synchronize command to the iSIC which overrides the upcoming sample and delays the taking of a sample until the next interval. Thus the current interval is skipped.
The iSIC RTC only drifts a maximum of 2 minutes a month. Due to this, it is unnecessary to synchronize the clock hourly. It is recommended that the synchronization schedule is set between samples once a week. For example, if sampling occurs every 15 minutes then a good synchronization time would be on Sunday at 3:07 AM.