This is a topic that has been previously covered here by my good friend Ken Puls, but it was just recently that I found this response from Ehren about how Power Query interprets xls files differently to xlsx files and it is certainly worth a blog post to cover this in more detail.
I highly encourage you to read this post if you use Power Query inside Excel or Power BI against Excel files either in *.xls or *.xlsx
Excel specialist turned into BI specialist using the latest tools from Microsoft for BI – Power BI. He is the co-author of ‘M is for Data Monkey’, blogger and also Youtuber of powerful Excel video Tricks.
He has been recognized as a Microsoft Most Valuable Professional (MVP), is a Microsoft Certified Professional (MCP – MCSA: BI Reporting), a Microsoft Certified Trainer (MCT), and is one of the international pioneers in Power Pivot, Power Query and Power BI.