Well, that's certainly unusual, to my knowledge. IIRC, you're in pretty far west Texas, where you might think most criminal defendants are of limited means.
Oddly enough, with a smaller client, billing by the hour might tend to align costs with objectives better than an all-inclusive flat fee. But there's a big risk of non-payment if the work is billed after it's done.
Using bail as an example. Under pre-reform schemes, there might be a sort of standard amount of bail in a court/jurisdiction for a certain crime that tends to be imposed in a cookie-cutter fashion, as in "oh, agg assault, that'll be 50k, please." And opposing that bail or asking for a reduction, might not be a very high-percentage of winning, absent some special facts. So prepping and making a bail-reduction motion takes four hours at $250, or $1000. While the bail itself is $5000. Could be very worthwhile if there are facts that support the reduction, but the attorney fees are sunk.
But if you have a $10k all-inclusive flat fee, a low-percentage bail motion probably gets ignored in favor of trial prep/seeking a plea.