Children's Tylenol - why do you have to use their cup?

This evening Frost developed a fever. At first it stayed low - around 100.5 - but by 10pm it had risen to 102.8. He was moaning as he tried to sleep and felt very warm so I decided to give him some Children's Tylenol to help him get some rest.

We have misplaced the cup supplied by the manufacturer. It went into the wash after the last round of diseases and it didn't find its way home to the medicine cupboard. However, I have many measuring tools from the days of Wren's oral medication and I happily measured off 10ml for Frost to suck up with my oral syringe.

That's when I noticed the big warning

"Use only the enclosed cup to give your child Children's Tylenol Elixir or Suspension Liquid. Tylenol products have been specially designed for use with the enclosed cup"

Does anyone know why or how? Do other measuring devices give wrong readings? Is 5ml of Children's Tylenol not really 5ml?

I started reading around a bit and found that people who are not medical professionals tend to measure things rather erratically and that measurement accuracy varies by method.

"CONCLUSIONS: Droppers and dosing cups were the most commonly used devices in the home for measuring liquid medications. Subjects were more likely to measure an acceptable dose with an oral syringe when compared with a dosing cup. However, a large proportion of study participants were unable to measure an accurate dose with either device. Community pharmacists should educate caregivers on the selection and proper use of measuring devices to improve the accuracy of medication administration in the home. "

[Accuracy of Oral Liquid Measuring Devices: Comparison of Dosing Cup and Oral Dosing Syringe Parisa Sobhani, PharmD et al]

As this study finds - in general the level of accuracy with a cup is less than with an oral syringe. So, what is it about this magic cup? Have they taken account of the viscosity of the liquid and taken into account the amount usually left behind when you drink from the cup? If so, shouldn't they caution the adult not to let the child lick the cup clean? Not to scoop or scrape every last drop from the cup?

I am just confused and need more information to know how to recalibrate my oral syringe to delivery the correct dosage of this product.

Frost is doing ok and is now asleep in bed. We will check on him overnight and hope that he feels better tomorrow and that Wren does not sicken with our latest scourge.