You didn't mention the effect power down and power up have on your voting power. The reason there are two numbers for our SP is because the one in brankets goes down if you power down and up if you power up.
A new Steemian has a voting power of 15 at 100% because he starts with 0.500 SP and a boost of 14.500 SP by the number in the brankets.
I currently aso have a voting power of 15 despite having an SP of 6.300. The reason is, I am powering down and thus my boost by the number in the brankets is not 14.500 but 8.700, for a total of 15.000. Thus, I still have 15 like that of a rookie.
The sp you are referring to that is in brackets is delegated Sp and has nothing to do with powering down as you can not take it out.
The only SP you can take out is your own.
then why does it go down when you power down, and why is the total always the exact voting power you have?
Delegated SP is included into your VP but you can't take it out. Because it's delegated meaning not yours and you got it from someone to use for a specific amount of time.
There is no way that you can take out delegated SP, only your own.
The delegation lowers by STEEM itself the more SP I gain while I power down.