private key – In SECP256k1 arithmetic, why is |a*b| – |a*(b+1)| very large?

First, I multiply two scalars on SECP2561 curve using secp256k1 java calculator:

E7D2AF2FC9FF9CAF795D2EED256567679873FC547A513AA85A8A2E2776AB88D9 * db09b0615ad40a0 =

fc07a1825367bbe

but then I add or substract from db09b0615ad40a0 just 1

RESULT

E7D2AF2FC9FF9CAF795D2EED256567679873FC547A513AA85A8A2E2776AB88D9 * db09b0615ad40a1 =

e7d2af2fc9ff9caf795d2eed256567679873fc547a513aa86a4aa83f9be20497

Why is the difference so BIG ?

What is the “magic” number ?