Why does your link your bank account form say 1 lac and 50,000?
The amount specified is the maximum amount and not the specific SIP you are planning at this time. This mechanism allows you to make multiple investments (either lump sum or SIP) without registering a separate bank linking form for each investment.
For example, when you make an investment of Rs. 1,00,000 and if your limit is Rs. 50,000, then the investment amount will be debited twice for Rs. 50,000 each which will be considered as one investment transaction for individual mutual fund companies.
Please note that the actual transfer from your bank account is as per the instruction you give us and not the upper limit specified in the bank linking form. The upper limits of amount have been decided based on the investing behavior of a typical investor.