I believe that this article is the best reference for your inquiry:
https://steempeak.com/steembasicincome/@steembasicincome/vote-values-in-steem-basic-income-part-four-value-balances-and-upvote-delivery

In brief, the rshares balance that is returned by our status call is your Current Pending Balance, without respect to what value you have already received (or what your earning rate is).

That balance grows steadily, increasing every 2.4 hours. Then when you post, you get an upvote of 20% of that (the 'next SBI upvote' predicted by the status call). When you increase your SBI units, the rate of balance increase grows immediately, but the balance itself will still take time to grow.

It's a little like driving from one city to another. You could step on the gas and drive at a faster rate, but if you check your distance after only a few minutes, you won't be that much closer to your destination than you were before. Measure again after a meaningful amount of time, and you will see that you are making faster progress.