Instant coffee has less caffeine than percolated and filter drip? Sources related to average caffeine content per preparation method
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for (6oz cup, arabica coffee)


Filter drip 150 [Mg. of Caffeine]
Percolated 125
Espresso 100
Instant   60 
Decaffeinated 4

instant coffee to have less caffeine than the methods involving filtering.
I just can't get my head around that, as in instant (e.g., Nescafe) coffee preparation we practically put the coffee into the cup and eat it.How is it possible filtered beans to have more caffeine than eaten beans?
With instant coffee, you are not "practically eating the beans;" you are drinking reconstituted dried coffee.

To create instant coffee granules, the manufacturer first brews the coffee using conventional methods and then either freeze-dries or spray-dries the coffee to create a soluble powder form. Dried coffee.

The amount of caffeine in the instant coffee will depend largely the original brewing process. Since commercial instant coffee is created in very large volumes, they are likely extracting coffee faster than our meticulous home-brewed methods. So while caffeine is water soluble, it takes more time for the caffeine to be extracted and absorbed in the water than other compounds. And "more time" is not something that the mass-produced instant process allows. Less extraction time = less caffeine.