This just in (Patreon)

 It seems that these patrons updated their membership values and monthly max after you had already created a paid post. Because they had already been billed at the lower amount, they were not billed for their new membership value for additional paid posts, as their total amount billed would exceed their new monthly max.

Example: [redacted] was pledged $20 (with a monthly max of $20) when you made a paid post on September 1st, and they were automatically billed for that post. They then increased to $30, and later $100 (all in September) with a monthly max of $100. If they had been charged for paid posts made after September 1st, they would have exceeded their new monthly max of $100 ($20 + $30 + $100 = $150, much higher than their monthly max!)

If these patrons did not have a monthly max set when they updated, they would have been billed their membership value for every paid post you made.

Moving forward, you can expect them to be charged their new membership value for any paid posts you make in October (up to their monthly maxes).