You're right about that observation. Our STEEM Power is always increasing. This is actually a consequence the Steem Blockchain's inflationary nature.
Holding STEEM Power actually gets you more STEEM Power. This may not be immediately obvious, especially for accounts with low STEEM Power, but the simple act of holding STEEM Power actually creates more of it. In fact, according to the Steemit.com FAQ, "STEEM tokens that are powered up to STEEM Power earn a small amount of interest for holding."
So, why does this happen?
We need to go back to the inflationary nature of the Steem Blockchain. In order to achieve its goal of tokenizing web content, the Steem Blockchain needs a continuous supply of tokens to reward its users. Thus, every year, new tokens are created according to the blockchain's inflation schedule, which since December 2016 has been set at 9.5%. However, as the Steem White Paper notes, the inflation rate was also designed to decrease by 0.01% for every 250,000 new blocks created on the chain, or at a rate of about 0.5% every year, until the overall inflation rate reaches 0.95%. Currently, according to Steemdb.com, the inflation rate stands at 8.65%.
How much increase can you expect?
The new tokens created as a result of the inflation rate will be added to the Rewards Pool, and distributed to all the stakeholders in the blockchain, with 75% going to authors, commenters, and curators, 10% going to witnesses, and the remaining 15% going to STEEM Power Holders.
Thus, using the current inflation rate of 8.65%, the 15% share in the Rewards Pool will amount to roughly a 1.298% APR increase for our STEEM Power. So, for an account that has 1,000 STEEM Power, that's about 12.98 more STEEM Power every year, or roughly 0.250 more STEEM Power per week.
So, you're right. Your STEEM Power increases everyday, as more STEEM are created by the blockchain. This is because of the inflationary model of Steem, which rewards STEEM Holders about 15% of the annual inflation.
Note: Please keep in mind that these numbers are just approximations. Also, if you want a more technical and detailed explanation of why our STEEM Power is always increasing, you can read the article by @lukestokes, which I'll link to below.
There are actually three cryptocurrencies on Steem: STEEM (liquid), Steem Backed Dollar (liquid, more or less successfully pegged to the USD), and Vest. In fact, there is no such thing as Steem Power. Vests are what Steem Power is under the hood. The exchange rate between Vests and STEEM changes at a constant but slow annual rate. What this means is that holders of Steem Power (or Vests) are rewarded for just for holding it.
In addition to @cryptocopy's great answer, there is actually a tool developed that lets you see in-real time the increase of one's Steem Power. The tool is called SteemDash and is under the SteemBotTracker website.
Or you can directly access the site in here: https://steembottracker.com/steemdash/?account=nonsowrites
As for you with a 243 owned Steem Power, it seems you are passively gaining 0.000002 every 10 seconds. Not bad! The more SP you own, the more the increase.
It is also worth to note that this increase only applies to powering up or Steem in Steem Power form. Holding Steem or SBD in liquid do not give you any increase.
@Nonsowrites, In my opinion you can call it as "Return On Investment", for holding the Steem Power. I may be wrong on the point of the naming as ROI but it's like that only.
Wishing you an great day and stay blessed. 🙂