COBOL numeric fields

I mentioned here something odd and confusing to me. So played with it and certainly can work around it. However I still find it strange that in IBM COBOL, at least this very old version, I can’t move a field defined as 999.99 to another field defined as 999.99. I guess the answer simply is that’s the rule. This is a very old compiler, so as far as I know it may not even work this way any more.