Understanding Waterfall Emails and Related Costs

Hi there...i'm new, still learning. That said, I'm wondering a few things regarding waterfall emails.

1.
When I initially ran, it said it was going to cost 3. I thought it's 2 credits to find an email using any of the vendors like dropcontact or datagma.
2.
2. It has an email from one vendor in a column and then in multiple other columns, it shows the email. I thought when it finds the email, it doesn't run the other software. Example: Rows 29 and 30...they have emails in all columns from several vendors.
3.
Do I pay for more than one email vendor if they found it? I don't think so...but, thought I'd ask.
4.
Lastly, why in the Work Email column is there no email, even though there are emails in the columns from the email vendors (dropcontact, Hunter, etc)? Example: Rows 23,29,30

Thx!