Stacking not always working

[webwakko]

My timeline has a lot of items for each different group.. but it seems that the stacking setting does not always work for all items.

In this image you'll see on the left that 2 elements on the same time are semi stacked, while the once on the right side items are not stacked.
The once on the right is the behaviour I would like to see.. so wondering why it's not working for all of the items