Fit at 600 nm:
When one considers data extracted at 600 nm—a completely reasonable wavelength to choose—the single rate returned is 5 s-1. The residuals give no clue that the fit is more complicated than we show.

Fit at 500 nm:
When one considers data extracted at 500 nm—again, a completely reasonable wavelength—the single rate returned is 5.5 s-1. Again, with no other information, we would confidently declare 1-exponential.

Fit at 450 nm:
This time, your alert should go up (but would it?) because the rate constant is not within the 5–5.5 s-1 range the other fits calculated, but is now at 6 s-1! What is wrong? The fit in all three cases—if considered individually—would seem conclusive. The rate is 5 s-1. Or 5.5 s-1. Or 6 s-1. But when they are put together, and if a knowledgeable kineticist is interpreting the results, the conclusion would have to be made that "More experiments need to be done!" But, oops. The enzyme is all gone. Or, the student has graduated. Or, or, or.