Google Colab's compute units are evaporating periodically!


I am using Google Colab and the available computing units that I had purchased and accumulated while subscribing in the past are being removed by Google over time after I stopped subscribing. Why is that?

For my instance, a few months ago, I stopped my monthly subscription(adding 100 compute units monthly) of Google Colab which I had subscribed to for a long time. At that time, I had about 340 compute units remaining. About a month later I found my compute units became 300 exactly, and then I used for short time and it became 293 compute units. And after that, some weeks later it became 200(!), and I use it a little then 19x, and then it also became 100(!), then it became 99 the, now it will be zero. And I also have another my account, paid colab, which has become zero units already.
Google Colab is removing my ‘paid’ compute unit, if sometime later periodically.

I already asked the local service branch of Google, but they told me I should ask to Google Head Quarters in USA.

It would be greatly appreciated if you could explain why google do this way. I’ve lost the compute units that I paid.
And even if this is actual Google’s policy, there need a proper reason. What do you think?

Yours sincerely,

Self reply, There exist expire condition. So this is removing periodically. OK, but I hope the expire date condition to be repealed or prolonged.