It may be...
...the difference between greater than and greater than or equal to. Calculated with great precision, your decision indices would appear in tenths. I am familiar with the argument in favor of flooring, but all my indices are rounded. The original version of PBJ had them as points of indifference, where the action at the published true count was of equivalent expectation for either action. While not precisely true, it was true enough.
Absolutely precise play is just not possible because the undealt subset is described by all count systems in abbreviated fashion and for some decisions the most crucial ranks have been weighted inappropriately.
Be aware the the original purpose of point counts was to guide the betting. Playing decisions were largely afterthought.