You would only get your calculation if there was no overlap and all cams have some overlap.
There are also some laws of physics that come into play.
The Americans use dynamic compression ratio as a target that guides the required static compression ratio that you would build an engine to and its so much more than just cam overlap.
It takes into account, stroke, rod length and angle, cam lobe vented and a hole lot more.
Yes I did read articles on it and no I dont fully understand it but it makes a bit of sense how you can run higher static compression ratios with high lift higher overlap cams and why you shouldnt use higher compression ratios with high lift low overlap cams