Power query data types and extracting values

[dicken]

Hi,
I have a query (no pun intended) regarding data type and the option to expand,
I have the following

Table.Group(#"Changed Type", {"Product"}, {{"Sales", each List.Sum([Units]), type nullable number},
  { "StandardD" , each Number.Round( List.StandardDeviation( [Units]),2) },
 {  "What Date" , each  List.Distinct(  [Date] )   }} )

Which allows me to expand, but if I had a data ; ' type text' in this case as it is just list of dates , the option is no longer available,
but it's automatically added upon expansion.
I'm just curious as to how the data type is affecting what's going on?

Richard.