When a campaign is set up with a manually defined audience, looking at the audience of a campaign may show a number of users which is higher than the number of users that were invited.
To see the number of users in the audience of a campaign, access the campaign, then choose the campaign management tab. On the left side menu section, choose "Edit campaign" and access the "Audience" step (#7 out of 8 - #6 out of & when multi-language is not activated) and look for "Number of Invited Users" field.
The difference is caused by several reasons:
System admins are included in the audience of ALL campaigns in their zone
A user which is part of the audience, submitted an idea with co-authors that were not invited to that campaign.
Please note that this can only happen with users that belong in the same tenant. A user can only gain access to campaigns in his own tenant.