I have two ES and have the same phenomenon. Many factors at play, but if I had to list them in order of impact my list would be:
#1....Sun Exposure
#2....Cultivar
#3....Hydration
#4....Amount of fungus or disease(cercospora and p.mildew)
#5....General health and age of the plant

yg

Having read a lot lately on the lack of Fall color this year, the plant colors up according to sun exposure heat and daily temps. The warm daily sun brings the sap into the leaves, with sudden temp drop of evening trapping sugars in the leaves. Cold doesn't allow sap to return to stems or roots of plant quickly, like warm temps do. The sugars in leaves will affect the color spots in leaves. Each leaf has all the colors, green, yellow, red, but what food the plant is getting to the leaf in sap, will determine which colors show the most.

Maple Sugaring in pre-Spring weather is sap rising in tree. You get best production of raw sap with very warm days, cold nights. Sap rises, flows into buckets or tubes to be collected for cooking down. You can only get the good, raw sap before the Maples bud. Once the trees bud, the flavor of raw sap changes, not sweet anymore.

Plants work the same way, just on a tiny scale. So warm day, sun exposure, frost or cold locations in yard, can catch the sap in leaves or protect plant to allow sap to drain back to roots in Fall evenings. Trapped sap in leaves will color the best, brightest.

Depending on location, some plants were brilliant this year while others were a plainer color all over.

Definitely not enough info to make a scientific conclusions, but here is my observation of 10 ES planted in four different lighting conditions:
a) 4-5 hours of morning sun the whole season around, from early spring till late fall. Fall leaf color: undistinct olive-yellow.
b) 2-3 hours of direct mid-day sun, bright shade the whole day before and after that. Bright sun the whole day in a fall. Fall leaf color: bright yellow.
c) less than 1 hour of direct sun in a summer, open shade the rest of the day. Bright shade the most of the day in a fall.
Fall leaf color: Greenish blue(not a bluish green!?) with just a faint of yellow on some leaves. This group will keep its leaves much longer than all other groups.
d) Fairly deep shade till noon, open, but not so bright shade from noon till 5-6pm, 2 hours of direct late afternoon sun. Almost the full sun in a fall. Fall leaf color: the most intense orange-red almost rivaling nearby 'Lady in Red'.

Based on this limited data (though performance of all four group is consistent for the last 3 years since planting) I could venture idea that the more drastic change in a lighting conditions plant incure in a fall vs summer the more intense fall color change would be.

History of my two 'Lady in Red' also confirms this idea,
in location a) it slowly transitioned from dark green to dark burgundy without a trace of yellow and red, but in location d) it went thru all stages of green/yellow/orange/red/burgundy display creating quite a show, though a short lived one.

For what it worth.