If you see Row IDs skipping numbers, this means some rows were deleted. The essence of Row ID is to be set once for each particular row and never change.
If you want to assign indices to row positions you can use the thisTable.Find(thisRow) idiom. This will recalculate on each added/deleted/moved row though, and those indices won’t be stable (i.e. may change per row).
If you want to hide the fact that row IDs are not sequential but still have stable IDs that uniquely link to a row indefinitely, try Row UIDs.