The "Earnable matured interest" in the pop-up box next to the Staking circle is actually the Time-active fraction with range of 0 to 1 (0% to 100%)
Please see pages 6 and 7 of the link below:
http://www.vericoin.info/downloads/VeriCoinPoSTWhitePaper10May2015.pdf

It is a confusing label carried over from VeriCoin.
An attempt will be to display the interest rate if it does not consume to much CPU power.
If not, the confusing label "Earnable matured interest" will be removed.