Why do I get charged for some International SMS but not all?

I live in Korea, and converted my Verizon service to eSIM (using Iphone 12), because I need to keep my US number due to my entire life being connected to it, 2 factor authentications, etc. I inserted a local Korean SIM card for local data and use a local Korean number to text local friends and make local calls. The MAJORITY of my friends/family in the US are iphone users, so I can text back and forth with them over imessage for free (blue bubble texts), and I always do that using my US Verizon number. There are 2 people in my life that I text regularly with, that use Android (and thus, the green bubble texts), and I also always text them using my US Verizon number. For the last year, I have noticed that my phone bill varies greatly each month and IF I am charged for incoming/outgoing SMS messages, it is to those 2 people mostly (sometimes the random 2FA text will show up on my bill). At first, I thought that SMS between android and iphone over wifi was always free, and that I must just be out and about and not attached to wifi when I send/receive these particular texts that I'm getting charged for. But the more I read about SMS, I now understand that SMS has nothing to do with wifi and/or cellular data. So now I am even more confused as to why not ALL of my text messages to these 2 users are not being billed to me each month? I have gone through each bill each month and there are some texts (both incoming and outgoing) that do get charged but A LOT that do not. Don't get me wrong, I am not complaining, but I'm just more confused about why this happens? If international SMS are not free (I do NOT have an international plan), why do I only get charged for some SMS and not all?? Could it have something to do with the Korean data SIM I also have in the phone? Any thoughts/ideas/opinions? I'm just genuinely curious at this point....

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