Different trailer but same tires 2100's same size. My tires say "D" load rating on sidewall and they are 65psi. 50 psi is generally a "C" rated tires if I an not mistaking. See if your tires have a load rating on sidewall if "D" I would agree you are reading 65psi. As for the sticker you said it mentions "D" and 50 psi? I do recall a thread earlier this year about some trailer stickers having this info incorrect, I do not recall if it was your trailer or even your trailer line
According to duro's website they make that tire and size in both a "C" and "D" rating with 50 and 65psi ratings respectively.
Yes the link is an IP address but it is real http://184.108.40.206/products/trailer/ds-2100.php