Mysql - Value limit for mathematical operations

Question

I am in doubt about the following situation:

I have two values not stored in table.

Valor 1: 9223372036854775806 (Onde este é o valor máximo de um tipo de dado BIGINT)
Valor 2: 9999999999999999999999999999999999999999999999999999999999999999999999999999999 (Um número qualquer gigantesco)

When performing the following operation...

SELECT 9223372036854775807 + 1

The following error occurs:

Error Code: 1690. BIGINT value is out of range in '(9223372036854775807 + 1)'

Already in the following operation, it works normally...

SELECT 9999999999999999999999999999999999999999999999999999999999999999999999999999999 +1

Upshot:

10000000000000000000000000000000000000000000000000000000000000000000000000000000

I ask you...

If the limit for BIGINT is 9223372036854775807, pq no error occurs in the second operation?