It's hard to explain but I'll try. I have several forms in marketo. I put a different hidden field for Lead source on each one. The lead source in Salesforce always ends up being the same.
Each landing page has its own form.
I have a hidden field on each form that says Lander [whatever the form is]
This is supposed to translate into Salesforce.
If you look at the landing page http://pages.datavail.com/alwayson-availability-groups.html then you can see this field IS there, but still, the lead source shows up in salesforce as Lander Rac.
If I look at the lead in salesforce, the lead source is always Lander RAC but the activity history will tell you which form they filled out.