Launch credits are spent first, before earned credits. Unspent launch credits are accrued in the CPU credit balance, but do not count towards the CPU credit balance limit.
另外系統對每個帳號有限制，每個帳號每 24 小時在每區有 100 次的贈送限制：
There is a limit to the number of times T2 Standard instances can receive launch credits. The default limit is 100 launches or starts of all T2 Standard instances combined per account, per region, per rolling 24-hour period.
New accounts may have a lower limit, which increases over time based on your usage.
所以就知道為什麼會緩緩下降了，在到 144 之前都應該是下降的趨勢...