Because when you're logged into Steam and you play AVA via ignite, ignite detects Steam, and links your AGE account to Steam, so when you launch AVA through Steam, it'll automatically sign into that AGE account, in the same way that ijji's REACTOR allowed only 1 account to be linked to a single Steam account.
When Steam is active and ignite detects it, regardless of what account is logged into ignite, and it overrides the account which is logged on.
E.g. Account A is linked to Steam acc A,
Account B logs into ignite, but Steam acc A is still logged on
ignite detects Steam is running, so AVA launches with Steam acc A (Account A's) details.