Field1=500&Field2=50000

I agree to pay $500 per year.
For a life insurance policy value of $50000.

I agree to a $500 deductible on this building insurance.
I agree to pay $50000 for this building insurance.

IF

The adversary can change any part of the context
(questions asked, fine print, fontinfo, text color, background color, GUI element position, etc.)

THEN

The value of the digital signature as evidence will be called into question

BECAUSE

A verified digital signature can be presented for differing versions of the same agreement.