There are many ways an end user can run into this problem. One of the most common causes that I have seen is when pasting data.
Enter the Datatype
Sometimes when data is pasted, no matter if it is in a number format from the original source – it can be pasted as text or in string format. Strings are great, they can help display data in a wide variety ways, including numbers that start with zero or multiple zeros.
What is happening is the same as if you were trying to add the value of two words:
“apple” + “apple” = ?
In other words, your numbers are plain text like the word “apple” above. To fix this problem, and create the functionality to add, subtract, multiply, divide, etc… you need to convert your numbers back to the number data type.
When working with data, it is important to have an understanding of data types. A clean understanding of data types will help end users better process, convert and manipulate data in any process. Here is a good high-level resource on data types: http://en.wikipedia.org/wiki/Data_type
Microsoft has a great article for text to number conversion covering a wide variety of situations: