This particular feature provides valuable information to users by indicating the number of instances when an open has been activated at a pace faster than what can be accomplished by a human.
It is important to note that this may be activated quickly due to a number of factors, such as anti-virus software or iOS privacy tracking measures.
Your subscribers still got the message, but because of these features their computer automatically opened the message even though they may have never seen it.
In the realm of business-to-business (B2B) operations, the frequency of such occurrences can even reach as high as 80%. In B2C, it's much less.