Why is the diligence of Yan Ti missing a horizontal line?

As calligraphy, the phenomenon that Chinese characters have fewer strokes or more strokes is allowed, because that's what the ancients wrote. Because the handwriting is good, later generations compete to imitate it. Over time, it has been handed down, including some words reduced and strokes added, and even some misspelled places (such as Wang Xizhi's Preface to Lanting Collection), which have been preserved as calligraphy or its works. It has become an established and recognized correct writing or writing method. For example, the diligent character you mentioned just now is missing a horizontal line, and there are other characters, such as the word "Liu" is missing, the word "Monument" is missing an apostrophe, the word "Ming" is written as "four" next to the word "Japan", and a horizontal line is added inside. All of the above are recognized from the perspective of calligraphy, not typos. But if modern people use these words in daily communication or official documents, they are all wrong.