- the project in question is an outlier (one with an unusual number of defects), or
- there is a mistake in the calculation.
Either way, to get comfortable with the regression equation and your calculations, I suggest that you calculate the residuals (actual defects minus predicted defects) for each project in your data and then inspect them, say with a graph or histogram. Since your R squared is a very strong 91%, most of the residuals should be small in relation to the standard deviation of the dependent variable. An R squared of 91% means precisely that
You can do this calculation to check you are using the regression correctly. Also, the average of the residuals should be 0 exactly.