Have you ever received a delivery of oil and wondered why it is lighter than you expected?  It has to do with specific gravity, and all oils are not the same.  Those fancy scales that tell you that they are "weighing" your product in milliliters are actually assuming you are weighing water which is the same weight as in grams.  If you are weighing something other than water this is likely to be inaccurate unless they are VERY fancy scales that let you tell it what product you are weighing.

Litres are a volume measure, which means they will take up a certain amount of room in a container.  Kilograms are a measure of weight.  Unless you are using water (which is the universal measure between the two) they will not be the same. 

Oil is actually lighter than water which is why it floats.  You get more oil to a kilo than 1000mls, but that also means that you get less oil than a kilo in a litre.  As mentioned before, this has to do with the specific gravity of the oil.  This is the amount the oil weighs as it compares to water.  Water is always 1ml weighing one gram.  1ml of olive oil is 0.915 so you will only get 915gm of oil in your litre.  Something like castor oil is heavier and has a specific gravity of 0.959 so you will get 959gm of oil to your litre.  As you can see, all oils do not weigh the same and can vary quite considerably. Not all liquids are lighter than water, with mercury coming in at a whopping 13.6kg to a litre