In general aviation aircraft, we are being told to calculate TODR (takeoff distance required) using AFM and TODR x 1.25. Why do we need the value with 25% added?
This is called a "safety factor", and I was actually taught to use 1.33!
When you look at the POH or AFM the numbers and charts they present for performance figures were invariably calculated using a near-perfect/brand new aircraft, with a highly skilled test pilot.
That club aircraft you're learning in may well be older than you are, the performance of the engine not quite what it was at zero hours - and let's be honest your skill on take off might not yet be up to the level of that test pilot.
So in almost every performance calculation you figure out you'll want to add some amount of leeway from the published numbers to account for the aircraft and/or your personal skill level - be that take off, landing, minimums on an instrument approach etc.