For trains over NCR, i.e BP mail, TATA/SBP-JAT, Lalgarh-Prayagraj, Triveni & others... the reason may be to prevent early arrival or simply to treat them as Non-SF type less priority train.
For trains to get 130kmph sanction, the Engineering & mechanical dept jointly has to conduct Two running tests. 1st is CRS monitored trial using OMS-2000 machine. If proven satisfactory, it is followed by 2nd test, called Osscillograph run (COCR). Usually there is no accurate guideline about the Power of COCR run. Asansol division conducted 2 such COCR runs, one powered by...
more... WAP4, another time with WAP7 . The clearance certificate mentions both WAP4 & WAP7/5 are suitable for 130kmph.
Now I'm not sure if SER used a WAP4 for its COCR, the existing videos shows WAP7. In such case either the Loco inspector verbally restricts the crew to go above 110kmph if WAP4 is attached. The same reason trains like RNC-HWH-PRR, PURI-SHM weeklies sometimes run at 110kmph.