Several post have mentioned, this and different levels of sugars.  Do we
know this to be true?  Is there actual data that shows levels of sugars
being different? 

Of course. All nectars are different in their chemical composition, the proportion of various sugars, the concentration of sugar, the conditions that enhance or inhibit nectar secretion, the duration of nectar secretion, etc, etc. 

An obvious example would be: eucalyptus nectar is so sweet that it feels sticky when you get on you, which is easy to do because if you shake the flowers the sticky nectar splashes out. Orange is so low in sugar that hives can fill up with it but yield almost no orange orange once it's dehydrated by the bees. 

PLB

Southwick, E. E., Loper, G. M., & Sadwick, S. E. (1981). Nectar production, composition, energetics and pollinator attractiveness in spring flowers of western New York. American Journal of Botany, 994-1002.

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