unrealistic pending compactions displayed

Hi there,

I am on 2.1.7. I am using DateTieredCompactionStrategy with the following parameters:
min_threshold 16
max_threshold 16
base_time_seconds 300
max_stable_age_days 0.05

Apart from that, nothing exotic. I dump an sstable of about 18M every 12 sec I would say.

Maybe 1 hour after creating my cluster, I can see that:
1. the number of SSTables has constantly been below 60.
2. Most of the time, I have between 1 and 4 pending compaction.
3. I occasionaly have a large number of pending compactions (30-40) for a short period of time (a few min)
4. I witness on 1 node 1610 pending compaction, on another node 367 pending compaction, for a short period of time (a few min)

Item 4 makes me thing there is a bug as I don't see how I can have that many pending compaction with less than 50 sstable.