I'll assume it relates to the fact Longstone have gone from 44 singles in March to 43 single decks now despite the service increase on the 32.
The bus that previously ran the 20 may just be acting as breathing room during the day in case a different single deck elsewhere needs replacing. For a while the 20 also ran between peak extras on the 38, though that seemed to stop in recent times.
For those interested at the before and after:
March: 16/7, 19-28, 30 (13), 66-95 (30), 172 (1)
Now: 21/2, 24-30, 32/3 (10), 66-95 (30), 197-199 (3)