Why 50 MB error occur while activating zero rental plan in airtel

Many users got an error while activating the zero rental plan on airtel,
but now we have got reason of this error, today if you want to enjoy
fully uncapped speed then you need to activate a small 3g pack, it can
be zero rental plan, but when you activate it, by sending 3g to 121, or
through any other means, it shows you that you have more than 50 MB data
left, you can activate 3g plan only when you have less than 50 MB data
left in your account. Now we are going to reveal the reason of it.