I see a difference between the number of web push notifications Successfully Sent and the Impressions. What could be the possible reasons?

The number of web push notifications that MoEngage successfully passes to GCM is what you see as Successfully Sent.

Impressions is the number of notifications that reach the user.

There may be instances when Impressions are lower than Successfully Sent.

This drop-off can be attributed to the following factors.

1) User didn't come online till the time of notification expiry
2) Browser is not running on the user's device
3) Failure in communication between GCM-Browser-OS
4) User has turned off notifications on his/her device

Funnel-wise analysis right from the stage you select your target users till the stage the users get the web push notifications is available under the Campaign Reports.