Yes. Simple as that. Because it varies. Show me a retail store that has the same number and dollar amount of sales per day. It's not possible
Have you been spreadsheeting your sales and traffic by day? week? month? hour? The more data you compile as you go, the easier it is to predict somewhat sales trends. Plus it'll give you somewhat of a general range and you'll know better when there is a statistically significant rise or fall.
I think we often forget that we're dealing with people, and people are fickle as hell. Impulsive, emotional, quick to judge etc etc. Sometimes sales are great and it's hard to explain. Some days they just suck and it's hard to explain. But it's rare to actually have a statistically significant deviation from established norms.