Hi Lolabibilove!
Unfortunately, this is a known issue of Patreon. Which is why I have a warning written on my front page:

You can learn more about why this is happening throughthis link here.

Basically, it's because once you pledge, you gain access to 2+ years of saved content, as well as the content released / being released on the month you pledge.

In the past someone could easily sign up for one month, and cancel their subscriptions before their payment got through. So, they would have access to a lot of benefits without ever paying for it. Patreon created a direct pay system where you are charged immediately for that month to prevent this.

Let's say you pledged for July, if it was a 30 days subscription system like you suggested, you would not only gain access to July's content, but August's as well. For a monthly subscription system, this is potentially considered fraud.

I hope this could help clear things up a bit.

If you still have questions, I strongly recommend contacting Patreon. Since their payment method / issues are out of my hands. I only use patreon as a creator, I am not  Patreon's owner. 💖