I also added GetBlocksToMaturity during this time. I set it at +20, intentionally.
Although you can
technically get to it before it reaches 120,
until everyone in the network has at least 100 confirmed
confirmations from the mined block, it cannot be spent.
Other nodes recognize it as spendable, but the
client will not. In order to compensate for any delays in the network. Yes after 100 confirmations the
mined block has moved out of the immature state by then, but the client will wait until 120 blocks
before informing you that the generation has matured.
The figure was fixed, from the beginning.
Prebitcoin, this did not exist.