Why is there a +1 added on some phone numbers?

[blz2]

What's the deal with some contacts adding a +1 the phone number --- causing my text messages to fail to that person? It's not saved in the contact that way. I thought it was linked to Facebook because it adds the +1 for some reason, but I had the person block me from their phone number and synced my Facebook accounts. Their number disappeared. I unlinked all contacts associated with the contact. Using the contact I created as a Phone Contact, I tried to send a text message and it still failed.

Incredible2 on Verizon sending to US Cellular

Thanks for any advice