Hi Sherry,
Its because your trigger is on an account. That means its going to associate to anyone who sits under the account.
Think of this way:
Trigger: Send a Welcome Email whenever an Account Type changes to "Customer"
Now by saying this I'm stating send an email to everyone under that account because I'm not associating it to a Contact field which would say only when a specific contact's fields change.
If it were a filter for what you're doing above it would say...Show me all accounts with "X" and it would bring up all of the contacts sitting under accounts with field "x". Like show me all of my customer contacts?
Does that make sense?
If you wanted to limit it to only certain contacts, you'd have to specify in a filter which contacts should have an alert sent on them. You won't be able to send an alert just on account details as Marketo is a contact centric system and needs to associate the alert to someone's record.
Hope that makes sense!