Fulfilled its role, but strange at the end of cylinder ...

Useful for birthday parties. I'm not at my first bottle. However, I found a strange phenomenon only with that one.
The last balloons with the end of the bottle does not come up at all.
Yet they were swollen as big as the others.
As if the bottle did not contain only helium, but also the air heavier, remains at the bottom of the bottle ...
I could be wrong.
It was perhaps unintentional.
In short, without proof I will not blame the manufacturer complete with some compressed air to save helium, knowing it could be seen that in the end bottle ... But I admit that the my idea crossed my mind.
It remains to be monitored so ...