(some) Fabs only reserve 2/3 of what would be needed too reach 100%.

e.g. Oil Power plant 24k4 prod (w/ PAX bonus). 50% use of heavy oil. min. distance to oil refinery 96, connected with 120km/h train. Would need 1525 (14k4*0.5/1280*1.2) heavy oil cap. but it only reserve 940 or even lower (counting up if empty)
-> Oi refinery is backing up with heavy oil, Oil Power Plant can't reach 100% or rather produce const electricity (effc only 80%).