When dragging multiple messages, Outlook for Mac shows 3 little envelopes even if you have just 2 items selected. 3 is not 2. (Note that the line here is not a part of the dragging icon.)
Update: In fairness, Mail.app does this too - but it’s less conspicuous. The fact that the background 2 envelopes are faded indicates that this could be a really big stack, or a really small one. Contrast that with Outlook above, which has three solid-looking envelopes.