Thanks Jennifer. I guess the explanation must be some combination of factors as per your description. By way of example, an email sent last month recorded 1,454 unique clicks on one of its links. The page it linked to had 937 article views since then, and was an entry page in 808 visits (707 unique visits). Filtering that by a parameter we added to the email links, those figures reduce to 724 and 639 respectively.
The numbers we're instinctively comparing are 1,454 with 639 - it feels like quite a big discrepancy? Does that look explicable to you, or does it feel like something's gone awry?
Presumably the unique clicks figure in the email should be accurate - am I right to assume IDs would be appended to URLs, such that unique clickers can easily be identified by the system and counted once per email?
Thanks again,
Toby