Hi - likely you are encountering the fact that the daily racecard feed includes the exact distance yards, subject to rail movements etc, and historic data often includes the rounded distance in yards or distance in yards corresponding to furlongs. They are usually small variations but will not always be an exact match.
If you want to join between both tables, a distance_furlongs variable is useful, as supplied in the *insights tables - see the schemas at:
https://www.betwise.co.uk/smartform/historic_runners_insights
This will make joining on distances between the tables easier.