Why might a person get widely varying scores on different online IQ tests? Online IQ tests often yield wildly varying results (say 60 - 150). These are actual values obtained by repeated measures. It is puzzling, because it's such a wide range. How can the same person be mentally retarded and a genius at the same time?
Firmin et al. (2008) tested the validity of a handful of online IQ tests by having college students complete IQ tests at three different websites and also complete a validated lab measure, the Reynolds Intellectual Assessment Scales (RIAS). They found that there were moderate correlations ($r$ values were around 0.4 to 0.5) between some of the online tests and the RIAS, though other tests did not have a significant correlation with the RIAS. Thus, there was some evidence that online IQ tests are measuring the same kinds of things as lab-based IQ tests, but the correlations were weak and some tests were better than others. 

In short: online IQ tests are widely variable in their validity, and even the good ones are probably not all that accurate.

Reference

Firmin, M., Hwang, C., Burger, A., Sammons, J., &amp; Lowrie, R. (2008). Evaluating the concurrent validity of three web-based IQ tests
and the Reynolds Intellectual Assessment Scales (RIAS). Eastern Education Journal, 37, 20-28.