This is a normal behavior of Magento for displaying products when category are Anchored. I don't know the exact logic behind it but, this is what i have observed.
1) Suppose you have a Main category
Office Chairs in your case. Now this category may be having three subcatgory
ie: Rolling chair, Steel Chairs and Wooden chair
2) Suppose you have inserted 10 products in each of subcategories and 5 in Main category ie:
3) Now when the category is not Anchored ie: Is
Anchor -> NO, The category
Office Chairs will only show the 5 products associated with it.
4) When you make this category Anchored ie:
Is Anchor -> YES It will show the products of the Main Cateory ie
Office Chairs and also the products of its child category ie 'Rolling, steel and wooden chairs'. This is how Magento behaves.
Conclusion: The Anchored Category will display all the products ie: Products associated with Main category and also the products associated to its children category.
I don't know the exact reason for implementing it in the manner it is at present. Hope others at forum can provide light on it.