Op amp output resistor value
It's because you're connecting it to a load on the output which draws non-zero current. The bigger the current coming out of your op amp, the bigger the voltage drop on that resistor. The larger the resistance, the bigger the voltage drop. Without seeing the rest of your circuit I can't say for sure, but I'd guess that you can do without R19 altogether.
At the risk of sounding condescending... V=IR. Do a basic nodal analysis of that circuit with a resistor load, and you should understand what's going on.
If you absolutely must have overcurrent protection, a better design choice might be a resettable PTC fuse. However as stated in TimWescott's answer, it's likely overkill since most op amps can handle being shorted to ground without frying.