Why is it CST and not CSE in IIEST Shibpur?

It is mainly concerned with a naming convention and there is really nothing to it. If you take a look back at the history, the Computer science departments began somewhere around 1982 at both IIEST Shibpur and IIT Bombay. Both the colleges began with different names for the same department and have stuck to it over the years.

If you are only and primarily concerned with this naming, this difference in the name will not have any effect on the course content. You can rest assured about it.

A similar instance is that of BESU Shibpur. They used to honor graduates of engineering courses and the marks they provided were in percentages. This was not the case with a lot of other premier engineering institutes like NITs or IITs where the B. Tech degree is given on the basis of the grade point system.

All these minor differences are something that you do not need to be bothered about. These will definitely not tamper with your career in any way. CST is just the same as CSE.