Underpitched but achieved full fermentation

My last batch I brewed was an Irish Red that I ended up adding a little organic honey to at the end of the boil so I would have a little higher ABV when finished fermenting. Ended up with a 1.063 OG. Pitched one pack of Wyeast Irish Red with no starter and fermentation went crazy for about 4 days before slowing down. After one week it was already at 1.020 and after three weeks in the primary it was down to 1.010 FG.

Then I ran across Mr. Malty and plugged my numbers and according to the calculator I should have pitched 2.4 packs of yeast without a starter. But I ended up with a full fermentation with just one pack, so why would it suggest two? Did I just get lucky?

I know underpitching can produce off flavors, but last weekend I tried one after bottle conditioning for 2 weeks and it tastes great. Letting the rest condition for 3 weeks total.

I'm no expert (clearly, look at my experiences with 1084 right below), but Jamil never says that the only way to get good beer is by pitching at the correct rate. The notion is that you're going to get the best possible beer by pitching at the correct rate with a full compliment of healthy, active yeast. Does that make sense?

under pitching doesn't always result in under attenuated beer. in fact, many times the beer will attenuate the same. the issue with under pitching lies more in the off flavors produced by stressed yeast due to a low cell count. if the yeast have to reproduce too much in the wort, they'll kick excessive esters, diacetyl, etc.

Attenuation and pitch rate are not highly correlated. It's common to still get full attenuation when under/over pitching.

As you noted, flavor is probably the biggest factor for proper pitch rate. Still possible to get good flavor underpitching (and some high-ester styles recommended). In this case I bet you have tasty esters. But you get the best/most consistent flavor with proper pitch rates.

Sometimes it works out with just adding the pack. I would suggest making a starter though. Out of the couple of brews i've made where i only added one pack of yeast, a few of them never dropped below 1.020. This could be related to other factors (temps) as opposed to yeast cell count, but i haven't had any batches fall short of the recipe's FG since i began utilizing a starter wort.
Congrats on the Irish Red! I've got a keg of Irish Red that has been carbing for 2 weeks. Going to tap into that bad boy tonight. CHEERS!

Thanks for the quick responses guys. I'm still new at this (only my 3rd batch) but I'm learning so much it's crazy! I'm moving from extract to all grain soon, pretty excited about that.

I didn't know about Mr. Malty until about a week ago and it is definitely a great resource. I plan to make starters for all of my liquid yeasts from now on.

A quick note: I fermented the Irish Red around 64-65 degrees for three weeks, then moved it to 70-71 degrees for 3 days. Might have helped clean up some of the off flavors from underpitching maybe?