A simple answer
Why can the same trip have different prices?
Because the same trip can be offered through different booking paths — under different pricing conditions.
Same trip. Different pricing paths.
Where you book, what access is available, what inventory remains, when you look, and which promotions are active can all affect the price shown.
That does not tell us why one particular comparison turned out the way it did — and it does not guarantee which price will be lower.
What should you do with that?
First, make sure the offers really describe the same trip and include the costs and conditions that matter.
Then compare.
A public price may be lower.
Another price may be lower.
The prices may be the same.
Different price does not necessarily mean different trip.
Similar-looking offers are not automatically identical.
Compare before you assume.
That's it. Now you know what to look for.