Well, I found the explanation, MultiBit transaction details and help should be updated.
The additional pick status icon tells the transaction derives from mining, so it must wait the next for 120 blocks to be confirmed, in addition to the usual 120 nodes confirmation.
bitcoin-qt includes a specific explanation in such transaction details:
Generated coins must mature 120 blocks before they can be spent. When you generated this block, it was broadcast to the network to be added to the block chain. If it fails to get into the chain, its state will change to "not accepted" and it won't be spendable. This may occasionally happen if another node generates a block within a few seconds of yours.
Also, bitcoin-qt shows such transactions amount between square brackets, to signal you can't yet spend it.
Marked as solved.
Piero