I have seen that happen to me on occasion and I can't say with any certainty why it happens. It could be caused in the mash without you knowing it. This accelerated method seems to work pretty good. When I brew a lager I leave it colder longer than 3 days, it's more like 6-7 days and then I bring the temp up slowly to @ 65F. I don't watch the gravity at all until I cold crash it. So it stays at 65 for 5-10 days. Maybe it needs more time, who knows?
How long did the extra cells set in the refrigerator before the second starter was made? How big was the starter? The answer to the question may give a clue as what happened, but I think it would have to be a severe under-pitch for this to be a problem. As long as the wort was aerated with pure oxygen, the yeast can handle an under-pitch. This seems more like the yeast was stressed a bit, but from what it's hard to say.
If you taste the beer and it's doesn't taste sweet, phenolic, fruity or has a lot of sulphur, then the answer is likely in the mash. The mash can add some sweetness, but most sweetness comes from the yeast depending on the strain.
As long as the beer tastes good, then don't worry about, you shouldn't fret too much over a beer that didn't attenuate like you planned. Make note of it and if it happens again try to see if there are any similarities to might give you a clue as to why this happened. I don't get super attenuation with lager yeast, 76% is the upper limit most times. I had a Dunkel finished at 1.018 and it wasn't sweet, so the numbers don't always matter.
Hope that helps.