Principal limit is a unique term with the reverse mortgage and simply translates into how much you can borrow. Assuming the same data is used this number should be consistent from lender to lender. The data includes three critical numbers: age of the youngest borrower, rate, and home value. Here is a brief explanation of how the principal limit is calculated.
The age considers the youngest borrower or eligible non-borrowing spouse. Older borrowers generally qualify to borrow more or a higher principal limit. Younger borrowers generally qualify for less. As a side note, the age factor is based on actuarial age: living to 100 yrs. old.