Valganciclovir: Ranbaxy files post grant opposition

Mint reports that Ranbaxy has filed a post-grant opposition to the Valganciclovir product patent [IN207232].

Cipla which had earlier stated that it would lodge in a post grant opposition has. as yet, not done so.

This news was posted on the genericIP blog, but I missed putting it here.

Sun opposes Risperidone related patent

Varun had posted this:

Mumbai-based Sun Pharmaceuticals Ltd. has filed a post-grant opposition in respect to Indian Patent No. 208191 issued to Janssen Pharmaceutica N.V. against the mail-box Application No. 188/CAL/1995. The opposition was filed on January 02, 2008 with the Mumbai Patent Office, which last year in July (dated July 20, 2007) published the grant of the ‘191 patent under section 43(2) of the Patents Act, 1970. The ‘191 patent relates to sustained-release micro-particle of risperidone.
After Varun put his post, the media also covered it: here.

My query:
What has got me thinking that if the patent application was filed at Kol patent office, then how was it granted by the BOM patent office?
Once an answer to that comes across, then we can understand the reason behind the Post grant Opposition being filed at the BOM patent office?