Difference in Message Count in of Adapter and Integration Engine

I am new to SAP PI/XI and while working around with monitoring phase of PI I have noticed a unusual discrepancy in message count of Adapter and Integration Engine.

In my scenario, I have a Non-SAP system as Sender System thus an Adapter Engine at sender side comes into picture.

Now when i count number of messages in Sender Side Adapter Engine, I got a figure of 17975, indicating Sender has sent 17957 messages to PI.

But on other hand Count of in Integration Engine was Successful Messages: 15711 and Failed Messages: 36. Thus totaling 15747.

Now my doubt is if Sender Sent 17975 Messages how only 15747 are processed in Integration Engine. As per my understanding the count must tally or there must be data loss in PI which is very unlikely.

Your opinion will be appreciated.

Thanks in advance.

Regards,

Ankit.