What do those babies that you have labeled as Single Rex look like now? They don't look Rex to me either. Velveteens (a Rexoid) have wavy (instead of curly whiskers), but their coats are usually more wavy/curly at that age. Satin can be almost impossible to tell apart from standard at a young age (besides the slightly wavy whiskers and longer hair). If they don't look Rexoid now, they could indeed be Satin. Satin is a recessive coat type. It's not very uncommon, it's just usually misidentified.

Velveteen Whiskers.