According to TRAI rules, An ISP is bound to provide 80% of the promised speed from node to the user, That means From ISP end to user.
If they're able to provide that then everything is fine.
Elaborating your issue, Its common with everyone, Ask anyone who has 100Mbps shared residential plan and they shall they say the same thing.
To make you understand, There are two types of bandwidth here - Domestic and International.
Since Domestic bandwidth prices are almost 30% of the International pricing, These companies do the false advertising Saying that you will Get 100,200 mbps and what not. Because most of them check on Local Speedtest servers.
Most of these ISP's use Extreme peering Setup, Meaning popular content is cached or peered like Google services, Hotstar, Netflix or Amazon.
So most likely you'll end up getting Full speeds to these sites.
In Short, Majority of the ISP's do not have International bandwidth and they manage away with Domestic bandwidth.