Increase speed in RMAN channel

Hello,

In the RMAN we backup in 4 channel using infiny band. Channel 1 speed 120000 kb/sec, channel 2 speed 110000 kb/sec, channel 3 speed 115000 kb/sec and channel 4 speed 130000 kb/sec. If the channel 1, 2 and 3 finished backup, why the speed in channel 4 a little increase maybe just only 170000 kb/sec  not more?. Any parameter setting in netbackup or oracle?.

Thank you

Zainal