It's because the bombs are multiplexed hardware sprites and the timing is very precise. I only had four sprites free so when there are more than four bombs on a line the game is moving sprites left and right at the exact moment to appear like more than four.
Whatever emulator this is must not have completely accurate timing, which is a pity. I recommend using ACE-DL. I can't fix it because it's the emulator which has the problem. On real hardware the timing is correct. Please take it up with the emulator author - maybe they could use this as a benchmark...?