Why would excel add escape char (\) in front of space in custom number format?

Excel generated using custom number formats appear correct in \xl\styles.xml.

"<numFmt formatCode="# ##0;-# ##0" numFmtId="177"/>"

Here we have space character used as thousand separator and locale/language for Excel is French.

Numbers are not getting formatted correctly.

e.g. 1648975541 appears as 1648975 541

It should have been this : 1 648 975 541

When cell's number format is examined, it is showing as this: #\ ##0;-#\ ##0

Why is Excel adding escape char in front of space? Why does space character even need to be escaped?