a Potion of Swiftness II get transformed into an Potion of Swiftness I when you extend the effective time

I'm using Windows 7 x64, Java 7 Update 11

I expect when i put Redstone in a brewing stand together with a Potion of Swiftness II i will get the extended Potion of Swiftness I.

I my opinion we should get after adding some Redstone to any kind of strong Potions a Potion with the increased effect and not a weak Potion with an increased effective time.

Steps to Reproduce:

1. Put Waterbottles together with Netherwarts in a Brewing stand.
2. Let them brew and put Sugar into the Brewing stand
3. let them brew too, put some Glowstone into the Brewing stand an let it brew to increase the effect of the Potion
4. put some Redstone into it to increase the effective of the Potion.

And now you will get instead of a Potion of Swiftness II (4:00)
a Potion of Swiftness I (8:00)

In my opnion it's a Bug that will have impacts to the future of Brewing Machines in 1.5.