Shell File descriptors

Curious as to why file descriptors >= 10 are 'echoed', as shown in the examples below, whereas file descriptors < 10 are not.
Bash:
root@xt:~ # echo 10<&-
+ echo 10
10

root@xt:~ # echo 9<&-
+ echo


root@xt:~ # echo 8<&-
+ echo


root@xt:~ # echo 11<&-
+ echo 11
11
 
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