There is no adequate explanation for this, but the reverse of the 1922 British Penny has a very similar, but not identical reverse to the 1927 penny, a coin minted 5 years later.
The area of interest seems to be Britannia's index finger which is a little shorter.
Many coin catalogues point out that there is a difference, but do not go on to explain it.
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The British 1926 to 1927 Pennies had a slightly modified die from that used on the 1911 to 1926 British Pennies. The hair line is slightly different.