Not exactly. It means that -91 net posts were "created". Net posts are: Created – deleted +/moved. It is also subject to forum reindexes, which may affect these counters (data is derived from forum official stats, comparing consecutive months).
Essentially, it means that there have been more posts deleted or moved from the Indonesian Altcoin board, than created.

When this happens, normally due to an increase in moderation, it’s best to discard the month’s reading for the board. If it happens sparsely, it should not be much of an issue. If it happens regularly, as it does lately on the Russian Local board, it renders the post count unusable/unreliable for that board for the implicated months. It could also lead to asking the local mod, just to understand better what happened for the local language community.