I think Don is onto something here - I am so glad when he covers for my ovesights. I totally spaced on the pigtail thing - so we are both on the same thought about it possibly a three way or three-pole switch and your new one is single-pole.
(FYI in case terminology is getting to you here - 3-way or 4-way means either 2 or 3 different switches at different places control the same circuit - commonly at opposite ends of a large room, or top and bottom (and maybe middle of split-level landing with 4 way) of stairs. Single-pole means a single switch with one setting - on or off, for one item. Two-pole would normally mean Item 1 (maybe light) on, Item 2 on (maybe fan), or both off. Three-pole would normally be Item 1 on, Item 2 on, Item 1 and 2 on, or all off at the four settings. Just to make it more confusing, people tend to interchangeably uses the terms "way" and "pole". Also, "pole" and "throw" are interchangeable.
Another possibility comes to mind - especially if a dining room or high ceiling living room or such - one of the box blacks may be incoming live (to connect to the switch), one the live wire out from the switch to the light, and the other (red) live from the switch for a fan - the black and red outbound leads both going to the same place but one for fan, one for lights. White presumably is spliced white to white and tucked back in the box somewhere.
If you did not disconnect the red wire from the original switch - only a black, and you only have a light or a fan but not both, the red may be from when it had a fan and light both (or was at least wired for it) and used a 3-pole switch but unused now.
Confused yet ?
As for the photo - if available- need one of both new and old switches, or make and model number of each, identifying which is old and which is new.