Odd Language #110: Might Not or Can Not?

From a posting in USENET’s alt.folklore.computers newsgroup: ‘Many employer-based pension systems require _not_ working (at least for that employer) for at least a year as a condition of starting to collect benefits, but the rules naturally vary by employer. For instance, many teachers become substitutes for a year, to supplement their pension, and then return to full-time teaching so they can “double dip”–but those extra contributions may not increase their future pension payments.’

The issue: What does the “may not” mean? Is it “might not” or “can not”? The difference could mean a lot of money.