I understood that. I just mentioned SC-SBC to indicate the speed limit for the entire stretch.
The mobile speedometers are not always accurate, and a speed delta of +/- 10kmph is quite common (on many occasions, even larger errors can be seen).
Also, locomotive speedometers are quite prone to showing wrong speeds due to lack of calibration. Typically, the speedometers in all locos (except the WDP4/G4 series locos which have a doppler speed radar for accurate speed measurements) calculate speed by measuring the number...
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of rotations of the wheel each second and multiplying it by the circumference of the wheel.
Over time, the wheels wear out and their diameter is reduced and occasionally new wheel sets are provided to the locos with the correct diameter. So, the speedometer has to be constantly re-calibrated to make sure that the correct speed is displayed. Unfortunately, this is not done too rigorously in IR, and sometimes the loco may be apparently overspeeding, but in reality, the LP will still be sticking to the speed limit according to his speedometer. But LPs have other means of estimating their speed, and pretty much strictly follow the speed limits.

"Even in sc kzj section only rajdhani gr and vskp duronto touches 120kmph"
** SC Raj, SBC Raj, YPR-DEE Duranto, SC-VSKP Duranto, SC-NZM Duranto, VSKP GR do their MPS which is 120 in SC-KZJ section for which it is fit for.
" it cant run at that speed constantly due to huge gradients whereas in bzj ngp section mps will be continuously at 120kmph"
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Constant MPS or not is not a matter, wherever TSR/PSR are there, they have to slow. But section MPS matters, which is 120.