Well my experience tells me that If your opted for 2MB and getting 3MB PROVIDED that the sync rate is 3MB or higher also then clrearly a mistake at PTCL's end.
But I don't think its possible if you've opted for 2MB and have the sync rate of ~2MB in router's page and getting 3 or 4 or higher then 2MB.
Its possible that, lets just say if you have 4MB package and the synce rate of 10MB in router's page, what will happen is that you will get 490 to 510KBytes/sec stable speeds (not the speeds which u check from download managers or direct downloads which actually is burst speed and cannot be labeled as stable speed) provided your line stats are very good or near perfect. but you will never cross 4MB in any case unless you opt for 6MB or higher.
Like I have 4MB with sync rate of 4605 D/1244U on router's page and very good line stats so I get 420KB to 460KB STABLE down speeds and 85 to 115KB up speeds on torrents.
My previous 4MB with bit poor line stats had 4096/1024 so it yielded 400/65 KB on torrents.
So having higher sync rates and good line stats will ensure that you will get maximum out of your opted package BUT you can't cross it any case. PTCL has made sure of that.