Can the number of contacted leads in a campaign vary?

You can see the number of contacted leads in the Campaign Details screen of every campaign.

The number may vary because many contacts that had their status changed to 'Bounced' after having received a few emails are subtracted from the total number of 'contacted' leads.

For example, an email account that would have been counted towards the total number of people contacted had their email bounced later on in the sequence (likely due to being blocked); at that point they were removed from the total number of contacted leads.