If the tempo modes were the same, there wouldn't be a difference
In classic tempo mode, the duration of a tick is always the same. In your mod you only use 3 ticks/row which makes the track go twice as fast as normal (normal=6 ticks/row)
In modern tempo mode the number of ticks/row doesn't affect the duration, it changes the duration of the ticks instead of the row. This way you have to double rows/beat to get twice the number of rows/beat instead of halving the number of ticks/row.
So in short, the reason you're confused isn't because the modern tempo mode is confusing, it's because the classic tempo mode is confusing!
I noticed that you wrote mix mode in the last paragraph but I assume you mean tempo mode