This seems to be a silent rule for students arriving too early from south asian nations. CIC won't declare it officially to safeguard their political interests. They would let it spread mouth to mouth via consultancies. CBSA will deny entry to few students (specially the one who can't really explain their early arrival) to set an example, then the denied students will call up their consultancies to inform and then rest of the lot will be automatically taken care.
In a way it is good. What business a student has arriving 3-4 months before the program starts. I can understand 1 month, but 2/3/4 months is not logical. There may be corner cases, where certain student has a distant relative and may wish to attend some family function etc.