I'm French and I bet that it's the rythm. I can hear this foreigner with a perfect accent but with a way too perfect rythm with the same tonality:
"Bon-jour-deux-croi-ssants-s'il-vous-plait"
A French would sing it.
Bonjour ! : High pitched, the "bon-" louder than the "-jour", quick, dynamic.
A pause...
"Deux croissants" medium pitch, without any pause before:
"S'il vous plaît". Sometimes said very fast, since it's something you say everyday ("Silouplai"), and with a low pitch since it's the end of the sentence.