Recently, there was a conky update in Debian (sid):
conky (1.11.6-1) over (1.10.8-1.1) ...
With the new version, I noticed that the RAM utilization is different compared to what was in the previous version. Take a look at the following picture:

So free shows 816M used, but conky 1,2G.
At first, I though about the no_buffers option, but it's set as usual to true .
So why does conky show different RAM usage compared to free ?
Same here in Arch Linux conky shows increased RAM usage (by about 260MB) for quite a while
Same here on the lastest master commit.
My instance shows nearly 1GB difference, 4.9 (actual) vs 5.8 (conky)

Same here, in openSUSE Tumbleweed, Fedora, Mageia 8 (_beta_), Debian testing and Void, after July 2020 ─ but I have finally understood it is not an issue. ─ It is just a new and more acurate way to calculate "Mem used", proposed / subscribed by Linus Torwalds back in 2014 (_Kernel 3.14_).
It has been proposed to Conky back in June 2019 and applied in October 2019, about 10 months before Conky 1.11.6 landed to my rolling release distros and Fedora.
I am not an expert, and maybe I am wrong. ─ That is what I guess I have understood.
I've made a pull request #1028 for conky to have the same output than free.
The issue persists yet in Arch Linux , returns incorrect value for used RAM compared to htop and free -h.
kernel : 5.10.7
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I've made a pull request #1028 for conky to have the same output than free.