Most likely this. I'm not sure why card readers in Germany don't ask for the PIN every time but when they don't you're generally asked to sign the cashier's copy of the receipt and they'll verify the signature against what's on the card.

However not only will the cashier generally let you simply put the card next to the receipt while you sign (because this makes verification easier for them) but it doesn't seem like they actually apply any scrutiny: the signature on my card is very different from the one I use for signing these days and it never raised an eyebrow.

FWIW, I've signed for card transactions in all kinds of places across Germany, from small shops to large hotels. I can't figure out what triggers the decision between PIN and signature but I swear I've used either in the same place at different times for equivalent amounts.