A quick check for the options of NSolve yields:

 Options[NSolve] 

 {Method -> Automatic, RandomSeed -> 0,    VerifySolutions -> Automatic, WorkingPrecision -> Automatic} 

Does anyone know what ‘VerifySolutions -> Automatic’ means in this case? Automatic to me usually means ‘make reasonable guess’, as in ‘PlotRange -> Automatic’ but I don’t know what Mathematica thinks is reasonable, and I don’t know how to dig any deeper into the underlying code in Mathematica.

I should add that I know how to verify solutions by hand by feeding them back into the original equations. I also know that I can force the option by setting VerifySolutions -> True. I have found that this typically takes a bit longer for the equations I am solving which makes me think ‘Automatic’ means False in my case. But in either case, I am none-the-less still curious about my original question.