rat 5,
we operate our b777 with 3 derates as well TO, TO1 & TO2. TO1 is 10% less than TO and TO2 20% less than TO. It has all to do with economics. as mentioned by mutt, a 25% deduction from TO2 (ie. 20% reduced) is actually 40% reduction from full TO as opposed to only 25% had no derate options were given.
as Vmcg is govern by your derated NOT your reduced thrust, therefore it can be more favourable in a contaminated runway condition as less yaw is produced if an eng fails (unlike a reduced thrust situation still allows one the option to open up the power to full TO if an eng fails thus causing more yaw and increasing Vmcg)
SR