Over the last twelve months, ZynAddSubFX has seen a substantial decline in development activity. This could mean many things. This may be a warning sign that interest in this project is waning, or it may indicate a maturing software base that requires fewer fixes. It's also possible that development on this project has branched to a new source control repository somewhere else.
Ohloh makes this determination by comparing total number of commits made by all developers during the most recent twelve months with the same figure for the twelve months before that. The number of developers and total lines of code are not considered.