wiki这样解释了：“Early in 1993 Excel became the target of a trademark lawsuit by another company already selling a software package named "Excel" in the finance industry. As the result of the dispute Microsoft had to refer to the program as "Microsoft Excel" in all of its formal press releases and legal documents. However, over time this practice has been ignored, and Microsoft cleared up the issue permanently when they purchased the trademark of the other program.[citation needed] Microsoft also encouraged the use of the letters XL as shorthand for the program[citation needed]; while this is no longer common, the program's icon on Windows (before Excel 2013) still consists of a stylized combination of the two letters, and the file extension of the default Excel format is .xls (.xlsx in newer versions).” 所以就是说xl读音接近excel，另加s表示较新版本，加x表示更新版本，目前大多都是xlsx了。

XSL是指可扩展样式表语言 (EXtensible Stylesheet Language)，是一种用于以可读格式呈现 XML 数据的语言。

原来如此

谢谢，但您看错标题了。我问的是“xls"，不是你说的XSL。

XL [简明英汉词典]
abbr.extra large , 特大号