The SRTs should be rounded to account for this. Occasionally SRTs are found to be insufficient but usually Adjustment Allowance {} is added not performance allowance to make it clear to planners the time is there for actual runtime not to protect the timetable. Similarly when terminating on a half-minute, SRTs are rounded UP to the next whole minute with adjustment not performance allowance (usually).
On BIG difference with Adjustment over Engineering, Pathing and Performance allowances is that the latter 3 are all considered recoverable in one way or another (and often perhaps misleadingly are given the name 'recovery time', making people think they're all about performance), whereas Adjustment isn't, as it might say be to allow for a slower speed when approaching a diverging junction, or coming off a branch.
Importantly as a result Adjustment allowances don't show in a TRUST schedule as they're effectively part of the section running time, but the others do. Those 3 are then expected to be recoverable if the train is running late, so for example if a train is 4 late at one timing point, and 4 late at the next but had minutes of various 'recovery times' between the two it would have lost time against what it was expected to do.
That sometimes confuses people, but it's not half as bad as what it does to many computer based signalling systems such as ARS. It'll see the 3 allowances as recoverable even if the train is on time and can trick it into routing a train out at a junction early if it has, most commonly, pathing time approaching it. That's why planners should to be aware of how an area's signalled and use stops at junctions (even a . (dot) stop- effectively zero minutes will do) instead of pathing in ARS areas, though adjustment can sometimes be used to a similar effect.
First place I ever noticed it being a regular issue was at New Street maybe 25 years ago, but it happens everywhere now.
