My guess is the UI reflects cache policy, the cache policy is biased toward LRU and that LRU tends to correlate with age. To the degree my guess correlates to

My guess is the UI reflects cache policy, the cache policy is biased toward LRU and that LRU tends to correlate with age. To the degree my guess correlates to reality, older videos will tend to be in cooler storage and therefore more resource intensive to retrieve and load and play. And the UI change reflects engineering considerations as is the cultural practice at Google.