Saigas are very customizable weapons, so to an untrained eye, you might not recognize two identical models if they have been modified with different stocks, grips, scopes, sights, etc. The identifying characteristics that remain the same are not the most obvious ones that draw the attention of your eye. For example, if you look at the photos of those 2 very different looking Saigas, you can see they still have an identical and distinct geometry between the front grip and the shoulder stock, if you ignore all those modifiable features.
Now, even from the fuzzy video of them finding it in the trunk, it was recognizable as a semiautomatic, tactical-style shotgun, most likely a 12 gauge. The hardcore gun experts who are good at picking out the little details suggested a few different candidates, but by far, the majority pegged it as a Saiga. Turns out they were right.