Average visitor spend is a helpful metric to see how much visitors are spending at a given time, category, or city.
The math for average visitor spend is total spend / total cardholders. If you are looking at three days and one cardholder spends across all three days, the average cardholder spend goes up because the visitor spend goes up, while the cardholder count stays the same.
In theory... I am in New York for 4 days- November 30 - December 1 (spanning 2 months). I spend $250 a day, or $1000 during the trip
If you look at my cardholder spend on a daily basis it is $250
If you look at the cardholder spend by month it is $500
If you look at the average cardholder spend for the year it is $1,000
Basically, any time you see a smaller time frame, the average visitor spend is going to go down.
To take it one step further:
If 10 different people each day spend $10 each, that's a $10 avg daily spend by visitors, but in a 30-day month those 10 people continue to spend $10 per day, which is $300 average spend for the month. So when looking at a larger time period the average will fluctuate as new cards are swiped in market - it's not that everyone spends $10 per day.