Error statuses not consistent

[timur-g]

I was using timeout in bibisect script where "good" was hangup CPU ( --detect-hangups ) and "bad" timeout CPU (set at 75 of CPU). Result of a script is error code.

In order to have a different error codes, a used (renamed) timeout with --confess :
tmt --confess --detect-hangups -t 75 command param

But somehow results were not consistent, Hangup and Timeout had mixed error codes od 137 and 255, instead of different ones.
HANGUP CPU 6.40 status 255
TIMEOUT CPU 75 status 137
TIMEOUT CPU 75 status 137
TIMEOUT CPU 75 status 255
TIMEOUT CPU 75 status 137
TIMEOUT CPU 75 status 255

Please explain.