Your data does not show them to be common - less than 1 in 100,000 computing devices seeing an issue during a 7 month test qualifies as "rare" in my book (and in fact the vast majority of those events seem to come from a small number of server failures).
And we know from Google's datacenter research[0] that bit flips are highly correlated hard failures (i.e. they tend to result from a faulty DRAM module, and so affect a small number of machines repeatedly).
It's hard to pin down numbers for soft failures, but it seems to be somewhere in the realm of 100 events/gigabyte/year - and that's before any of the many ECC mechanisms do their thing. In practical sense, no consumer software worries about bit flips in RAM (whereas bit flips in storage are much more likely, hence checksumming DB rows, etc).
[0]: https://static.googleusercontent.com/media/research.google.c...
