I believe they could not or better said, they would not discern minor differences as inconsequential to record keeping.
In writing fractions the script of 2/3 is more exact than the decimal variety of 0.6666... or to round off 0.6667.
It is my opinion the Mayan calender became so ungainly was due to the fact they did not accept the idea 2/3 was a greater accuracy than an infinite decimal. Therefore, their record of time was far more intricate and accounted for minor differences that we have found to be burdensome to our science of Earth.
I believe their thoughts regarding the science of Earth Time was very sophisticated, but, their written expression was burdened with too much detail in fear of missing vital information in a growing knowledge base.
We know December 21st is a pivotal time in the seasons of Earth. We also know there are some incredible events in Earth's history at that time of year. The 21st of December is a dynamic time of Earth's orbit and it makes complete sense the Mayan's would find the time when Earth's orbit was accelerated to be nearing an 'end time.'
And NO, I do believe the world is coming to an end. I do believe the Mayans were NOT as accurate as we are. They were burdened with too many decimals in their calculations and did not discern the level of error affiliated with these calculations.
If you don't understand that, I ain't gonna explain it to ya. Sooner or later everyone without detailed knowledge has to accept and trust those with it.
