I think the reason the two booking methods are showing the same price is because the final ticket you will get if you buy either of these is identical in every way.
The only difference is in how you enter the information to the booking interface and how it is displayed to you in the confirmation screen.
So I wouldn't sweat it.

The reason they are showing up differently is because when someone uses the multi-city booking process it is expected that they will be visitiing several cities and so have layovers greater than 24 hours at some points. In that case you may find the prices changing because the flights and layovers would be different to the most direct routing. The user interface is therefore showing you the details in the manner you entered them because usually that would make more sense. But the system behind it will automatically issue you the entire ticket according to the fare rules, and that will end up being identical.
If you want to play around, you can use the ITA matrix tool (https://matrix.itasoftware.com/search) and enter your journey as multi-city or a return journey and look to see the fares and the rules that are returned. It will show they are the same.