What the FW13 does with 60W vs 100W PSU is explained quite well here (Both SPR chargers):
In summary, a FW13 has been seen to draw power peaks of 88W, this causes the laptop to top up from the battery during these peaks.
If you use a 100W charger with the FW13, it will dip into the battery less during these peaks.
Also, the FW13 will only ever draw 52W from a 60W charger, and 88W from a 100W charger.
I don’t know why it sets those level limits.