There are several reasons. First and most important is that PID and FID responses are bulk readings of many possible contaminants, while lab results distinguish between different compounds. The second is that lab equipment are specifically calibrated to specific compounds and ranges. With field equipment, the concentration and type of contaminant can affect equipment performance. Additional field factors (like water vapor) can affect readings.
The real answer is that PIDs and FIDs are not designed to measure "concentration", and can not be compared to lab results even under the most ideal circumstnces. They should be used as screening tools only.
I agree with 4tuna, these instruments are best for preliminary field screening only. If you need to correlate the field screening results with the lab data, you could try to run several samples and plot the lab results vs the PID/FID readings. If you're lucky there may be a correlation that you can use as a rough guide during future field screening. There is rarely a correlation, however.
A better option would be to use an immunoassay kit specific to your contaminant of concern or a portable GC.
4tuna brought out the most relevant points - PID/FID readings measure all VOCs in sample, and have a different response factor for each compound. I've found even a properly calibrated PID will slightly underestimate chlorinateds. If your system is at a petroleum site, your PID/FID is probably reading higher than your labs, because it is responding to all the TPH compounds, etc. that are not specifically analysed for in a typical 8260.
However, PID/FID CAN be quite reliable if you treat the instrument as the sensitive detector that it is. Make sure it is properly calibrated daily (or even more frequently) and you should probably set your response factor to 0.5 - give me more details and perhaps I can help you out.
I agree with the others except one consideration may have been overlooked. The laboratory really didn't analyze your samples and simply made something up! This happened to me and many other professionals in my area. I could not understand why a site that was contaminated by PID, visually, and olfactory, was below detection limits by the laboratory. In reverse, I have set up land farms for soil that was clearly not contaminated as later monitoring depicted. I found out several years later, after the offending lab manager was fired, what the REAL problem was. Hopefully, your problem is as the others described, but it is prudent to resample (as I should have) if the laboratory results are too far out there.
