Interesting observation. I think the "under $25" that you typed in got interpreted by the website as "under $25 per month on the payment plan." I looked at the cheapest phone in the results you give, and you can buy it outright for $200. So it shouldn't have showed up as an "under $25 phone", but I do see what happened.
I don't know, I guess they just assume people are not going to pay the full price for the phone?
The amount does actually come out to the same thing, though. The advertised price is $5.55 per month for 36 months. That comes out to a total of $199.80 over three years, whereas the one-time price is $199.99. So, it's actually a better deal to pay over three years, if you figure in inflation and the greater value of money now than money later.