SFSpeechRecognizer not returning numbers

When I spea\k a number the recoginzer returns the actual number if it is greater that 10. If the number if less than 10, it returns the number spelt out. For example,

speak "twenty", get "20"

speak "six", get "six", not "6"

Is there a reason for this? Is it a feature that I can turn off? I really don't want to have to parse the text of some speech to look for spoken numbers below 10!