test of producer's delay and consumer's delay

[Kafka]

hello，I have done a series of tests on kafka 0.9.0，and one of the results 
confused me.

test enviroment:
     kafka cluster: 3 brokers，8core cpu ／ 8g mem ／1g netcard
     client：4core cpu／4g mem
     topic：6 partitions，2 replica
     
     total messages：10000
     singal message size：1024byte
     fetch.min.bytes:1
     fetch.wait.max.ms:100ms

all send tests are under the enviroment of using scala sync interface，

when I set ack to 0，the producer’s delay is 0.3ms，the consumer’s delay is 7.7ms
when I set ack to 1，the producer's delay is 1.6ms, the consumer’s delay is 3.7ms
when I set ack to -1，the produce's delay is 3.5ms, the consumer’s delay is 4.2ms

but why consumer’s delay is decreased when I set ack from 0 to 1，its confused 
me。