Questions about traffic.

I have some PCM lines with a number of traffic channels, and two scenarios:

-1st scenario:
BHCA = 1
Medium conversation time = 29 s
Traffic Channel Load = 0.37 Erl
Used lines = 25

-2nd scenario:
BHCA = 1
Medium conversation time = 90 s
Traffic Channel Load = 0.95 Erl
Used lines = 25

My limitation, as you can see, is the number of lines, which is 25.
Can somebody explain me, in a comprehensive way, why do I have to under-use the channel (only 0.37 Erlangs) in the first scenario (when the medium conversation time is very low)? The results are correct, but I cannot understand why.

Best regards,

Javier