General PSpice and 555 timer question regarding precision
A circuit diagram of what you are doing is close to obligatory  - 
wise men who at their end know dark is right because their words had forked no lightning still make obvious mistakes - the rest of us the more so. 

LM555 circuit diagram here

There is no reason [tm] that P-S.P.I.C.E.* cannot model a 555 like timer to have a high degree of precision if the SPICE model makers wish it to.

SO the fact that it doesn't either suggests that either:


They don't wish it to.
This would be the case if eg they considered that the 555 IC did not produce overly accurately reproducible results between ICs - you can look at the datasheet to see if this may be so.


or


There is a bug in your P-SPICE.


OR


You are using values or parameters that are outside the reasonable range.
1k &lt;= Ra &lt;= 100k 


What value of external resistors are you using?

555 accuracy will be affected by the internal threshold levels in the ramp up / ramp down window comparators which in turn depend on fabrication issues. IF the maker has chosen to make these accurate they can be so. If not then they probably wont be.

The LM555 datasheet suggests that the untrimmed accuracy of a '555 is "not too bad."


Timing error monostable mode 1%/3% typical/max 
Timing error Astable 2.25% tpical no max - suggesting just maybe up to 7% max based om monostable spec.


Timing erreor 150 ppm /degree C. (-.015% per C or 1.5% over 100 C range (simplistically).
Depending on actual circuit, IF your main power supply has significant noise significant errors could be introduced. 



The best bet so far is that you are doing something wrong.
 Next best is broken SPICE.  




Spice is an acronym so should be, at least, SPICE, if not acyually S.P.I.C.C.E.
Simulation program with integrated circuit emphasis, AFAIR.