Great Stat Splinter! That does seem to be unusual.
4 times all in March and no others at all
Nov 1698-March 1699 within 0.6
Nov 1788-March 1799 within 0.9
Nov 1922-March 1923 within 0.9
Nov 2019-March 2020 within 0.9
The 1698/99 one is a bit iffy as some of this was only recorded (retro-calculated) to 0.5C
The worst fit is June 1947 where the spread from Feb 1947 to June 1947 is 17.4C