I say this every year, but it's worth repeating to explain the problems facing the band's singers, when people criticise them.
Historically, songwriters earned their money from the sales of sheet music. This was because before WWll in the USA., the biggest market for sheet music, there were more pianos in homes than radios. In fact the "hit parade" was based on the sales of sheet music rather than those of records until the fifties.
If you were a songwriter, it would be a waste of time writing something that needed a vocal range of four octaves, if you were hoping people would buy it to, "sing around a piano" as was the practice in many homes before WWll, and from where would come your principal income.
So the range of the "top line" was usually always written within a range of little more than the average range of the regular purchasers, not much more than an octave and a half.
Think of most of the classic ballroom tunes from stage musicals, by Gershwin, Cole Porter, Rogers and Hammerstein/Hart, Irving Berlin, etc,, in fact think any Sinatra tune.
These the bands singers can sing well, without straining their voices, though most have a greater range than what I've mentioned.
You can't always replicate the stuff made in recording studios. Any studio recording can be changed to any key you like with the technology which has been available for a couple of decades.
Nor can the band singers always replicate songs where the original artist has an unusually wide vocal range.
It isn't possible. But the numpty Strictly producer thinks it is.