Block times vary, that's completely normal.
Sometimes it feels like it happens always when I'm about to do something, i was creating assets with those two blocks.
Surely it doesn't have that much impact on me when I happen to catch a 15 seconds block...
Me also observed 'longer time to block' then average block time when I broadcast any asset related transaction. It seems it needs more time when it need to add any asset (trade/transfer/create) related transaction on block...