So for example if you had 104 STEEM Power in your wallet and powered down, you'll receive 1 STEEM per week for the next two years. The price you can sell that 1 STEEM at will depend on the price of STEEM at the time you sell it. So this can fluctuate.

This seems completely false. From whitepaper:

Because Steem wants to encourage long-term growth, it is hardwired to allocate 9 STEEM to Steem Power (SP) stakeholders for every 1 STEEM it creates to fund growth through contribution incentives. Over time this drives the ratio of the total STEEM value of Steem Power balances to the total of STEEM balances toward 9:1 . (It seems likely that the ratio will be somewhat greater than 9:1 due to continued net Powering Up of the newly printed STEEM.) It also means that long-term holders are almost completely protected from the dilution used to fund growth.

SP can only be converted back to STEEM over 2 years via 104 equal weekly payments. ‘1 SP’ can be viewed as a share in a pool of STEEM. The network automatically adds STEEM to the pool every block. At any time users can convert their STEEM into SP at the same ratio as STEEM in the vesting pool to total SP. Converting STEEM to SP does not dilute existing holders of SP. Likewise, every time SP is converted back to STEEM it is done at the current ratio. Individuals are guaranteed to have more STEEM in the future than they have when they first convert from STEEM to SP.

So minimum ratio is 1:1 as you say, but can arrive up to 9:1 over time, even if I'm not sure according to what timing.