tr squeeze repeats does not remove multiple spaces coming from a piped du command

From: Zangune (Zangun...)

Greetings, I want to know the total size of a directory content, so I
use this command:

du -c /path | tr -s ' '

but I got this (this is just the last line):

510 total

please note that there are 4 spaces between "0" and "t".
I would expect the result to be:

510 total

please note that there is just one space between "0" and "t".
As far as I can see, the "du" command result is alined in columns with
spaces, but the "tr" command does not remove those repeated spaces like
it does in

echo 'a b' | tr -s ' '

Why doesn't a "tr" command with the "squeeze repeats" option remove
repeated spaces of a piped "du" command result?
Please note that I found a workaround, this one:

echo $(du -c /path)

but I am more interested in the theoretical problem solution (and
logic), rather than the practical one.

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