Variable OBA value question

[Michael Murphy]

I was playing Cassino-44's scenario 26 (Sant' Angelo) and I came across something puzzling. The German has to roll for OBA availability. On a roll of 1 he gets nothing. However, he gets 2x16 for a roll of 2-4 and 2x20 if he rolls 5-6. My question is this; what is the functional difference in these two artillery values? On the Bombardment Fire chart both calibers use either the 16 column for resolution of one OBA or the 30 column if you combine the OBA's into one attack.

Am I missing something here?