This circuit would do much better being operated from Vcc >=12V than from 5V. A standard 555 is less stable at near its minimum operating voltage. Your sim makes no allowance for any load current being drawn from pin 3 (Out) so I agree with Alec; that load current changes the pin3 voltage that goes into the timing network. You should put a buffer between pin 3 and the load...
Finally, the 555 model in the LTSpice misc library is an idealized "behavioral" model which does not account for Voh and Vol under varying source and sink currents in-to/out-of pin 3.
Get a "real" model of a specific 555 IC chip (TI bipolar or CMOS), link it into the LTSpice library, connect a load which mimics your load in parallel with the timing network, then the sim will match your reality. What you modeled, and what you built are two different things (as you found out)...