Are the save bonuses of some monsters in the Monster Manual (5E) miscalculated? Looking at the dracolich (pg 84) for example, I see that it's save bonuses seem to be calculated as ability bonus +5. However as its CR is 17, the appropriate proficiency bonus for that challenge rating should be +6, based on the table on page 8.
Shadow dragons (pg 85) similarly seem to be off too, with a CR of 13 but a proficiency bonus apparently being +4 rather than the +5 appropriate for CR 13 to 16.
Are there errors in the stat blocks, or am I misunderstanding something here?
So far, every monster I have checked—with the exception of these two—follow the expected correlation between CR and the underlying proficiency bonus in stat calculations. This makes me strongly suspect that they are calculation errors, based on CRs of 16 and 12 respectively. Perhaps the developers made a last minute fudge to their CRs without updating the stats, as this discrepancy seems unique to them. My initial confusion was that for some reason these two were the first monsters I looked at, as I was just starting to figure out how monster saves worked.