Cling!  Bottle Cling.  Much like static cling.  This is what I call the minute about of scent oil that stays in the bottle no matter how long you hold it upside down.  It really isn't a large amount of scent and overall is not really a big deal especially with larger bottles of oil.  With larger bottles there is a smaller bottle to oil ratio so the ratio of lost oil is smaller.  But when, lets say, there is only a .5 oz (1/2oz) of oil the amount that stays in the bottle is a much higher ratio.  That combined with the inaccuracy of most scales when filling the bottle can lead to as  a .125oz (1/8oz ) loss of oil or even a little more! 

Lets look at scale error.  Some scales are accurate down to the ounce some to the .1 oz and some to the .01ounce.  Very few in use are more accurate then .01oz.  What that means is the when a sample is weighed out to the ounce you may barely be getting an ounce or you may be getting 1.09oz.  (Depending on the scale).  Now if you want to use an ounce of oil for your candle and you pour it from the bottle without weighing you may actually only be pouring in .875 oz (7/8 oz).  This could really be throwing off your evaluation for strength of the oil and that is why it is so important to make sure that you weigh out everything directly in the pot or use a micro scale.  Again, if you use a Dixie Cup or other devise to weigh out your oil and pour it in then you will be losing some oil to "Cling". Especially when so little fragrance oil is needed for small candles.

I am sure that you can see that the error effect lessens as the sizes get larger. The effect of cling will be much greater if making a single votive then when making a batch of 100 votives (as long as the oil is mixed in at the same time)

Trying to evaluate sample bottles of oil should be used with the most caution.  For example if you are making a soy candle with a Super Concentrate (.6oz per pound usage in soy) and you are using a .5oz sample bottle you really need to make sure that the amounts will work out.  I will usually make 2 votives (4oz of wax) so that I only need to use .15 oz of oil.  That way I make sure that I have enough.  I do have a micro scale that weighs down to .01 with accuracy.

I use my micro scale by subtracting the weight of the oil.  First I put the little bottle of oil on the scale and "tare" or zero it out.  Then I pour the oil into the pour pot.  When I put the bottle back on the scale I can see how much oil as been used with the negative reading.  When it says -.15oz I know that I have enough oil for my two votives when using a Super Concentrated oil.

I hope that this has helped to see the value of weighing out your fragrance oil using reverse methods on a scale.  Especially for testing samples!