As others have mentioned... the lower range for that yeast would have put you at 1.016. Four points is not too far off, but there are many different variables to consider. This is mostly speculation.
1. Potentially unfermentable wort with all the dextrine, possibly higher mash for a bit; both could lead to troubles with fermentation.
2. Without rehydrating, you might lose 50% of your yeast, and I'm guessing that the ones left over are not exactly... healthy.
3. Swings in fermentation temperature would further stress the yeast, causing death... OR WORSE!
4. Fermentation is basically done at the end there, stopping at 1.020. Pitching more yeast might have little to no effect at that point.
All in all, no big deal. There are too many variables to get hung up over 4-5 gravity points. RDWHAHB