Halachah is not all black and white; rather, in many instances there is an entire parameter of correctness ranging from the very lenient to the super strict.

Some agencies, for example, insist that all dairy products be Chalav Yisrael—milk supervised by a Jew at the time of milking, while others do not. These organizations rely on a Halachic ruling allowing the consumption of non Chalav Yisrael products under certain circumstances.

Another example: In Europe, the number of people keeping Kosher is less than in America. Many of the Kashrut organizations there rely on dispensations that the Code of Jewish Law would consider only post facto kosher.

A plain K is not a guarantee of kosher

This saves the company money and would therefore make kosher more cost effective for them. These agencies realize that it is better to afford the kosher consumer a lesser degree of kosher than no kosher at all.  The kosher symbol represents a certain philosophy and approach to kosher—that of the governing board of rabbis for that organization.

Additionally, not every organization is trustworthy. Unfortunately, Kosher symbols are not limited to organizations whose primary concern is halachah. It is important to inquire regarding the validity of any unrecognized symbol.
 
A plain K is not a guarantee of kosher. Copyright laws do not apply to single letters of the alphabet and anyone can put a k on a product.