Here are a few of the whys:
The nutrients in the soil may alter the flower size and color, the height of the scapes, the bud count and the size of the clump.
How many years the plant is in your garden may influence the flower size and color. If it's the first full year your plant has been in the garden (in one place), don't expect it to be perfect. A clump that's been in the ground (and expanding) for many years may stop producing many flowers because it needs divided.
Bloom time usually has an influence on the flowers. The first blooms of the year may be the least beautiful. Plants that re bloom may have less stellar flowers on the second batch. Continuous bloomers may have a short period where they need a rest before starting again.
Moisture can help and hurt daylilies. Your daylilies may loose buds (prior to blooming) if your area is in the middle of a drought. The roots (rhizomes) may rot if they sit in water anytime, but, especially during the winter. They need regular moisture in well drained soil.
The amount of light they receive may alter the bloom look, the production and clump size. Some daylilies do well in partial shade. In the hot south, most need afternoon shade. Others may produce only a few scapes and blooms in the shade.
Varmints may decide the rhizomes are the next best thing to chocolate pudding and make them their winter meals. If you have a daylily totally disappear over winter, chances are it's a vole, mole, mice or deer issue. Assuming it's been healthy prior to this.
Insects can really do a nasty number on daylily flowers. It isn't the color because I've had the very same colored lilies with one having damage and the other perfect. Earwigs and Japanese Beetles will eat the color off some flowers once bloomed. Earwigs may also eat on the buds causing them to open with damage or to be disfigured. Flower thrips, spider mites and aphids will strip the buds, suck sap from petals and may damage the foliage. They are very tiny but they can kill a plant if infestation is large. Slugs and snails may munch on spring foliage. All can be controlled without using insecticides.
Some daylilies may be labeled the same name, but, I've found the ones purchased from big reputable daylily dealers to be the most consistent. Most daylilies from big box or other discount places may be labeled a certain name but often they are a little or a lot different. Buying from an ebay or other individual seller ,where you've never seen their gardens, may produce varied products.
The last and totally the most unscientific: daylilies have a mind of their own (so to speak). You may never know why one year is different from the next. Unless it's a disease or a preventable issue, go with the flow and figure you have two plants for the price of one.
The photos in this article are of "Designer Gown". The flowers look different at different times. There are gardeners who become really upset if a daylily doesn't have consistent everything. Any gardener who has paid a premium for a specific daylily will be upset.
Starting at the top:
A pale lavender pink with light purple eyezone and edges. Chartreuse green to yellow eye.
A pale flesh tone with a light wine colored eyezone and no edge coloring. Eye remains the same.
A true pink with a rose eyezone and edges. Eye remains the same.
A deep peach with a dark rose eyezone and edges with an eye of mostly yellow.
Last is a lavender with purple edges and eyezone and mostly green eye.
Honorable Mention Award.