This is the raw per-collection-interval delta for loss-of-synchronization events: it holds how much the underlying counter (counts how many times the receiver picked up an optical signal but could not lock onto the FC bit clock and word boundary needed to decode it) increased during the single most recent sampling interval, rather than the all-time total. For one specific FC port's current error-statistics object, watching this delta rather than the lifetime counter is what lets monitoring catch a fresh burst of errors quickly instead of waiting for a slow-moving cumulative total to visibly move. For example, a delta of 6 in one interval on an otherwise-quiet port is a much stronger signal of an emerging problem than the same 6 buried inside a lifetime count of 50,000.