What you have observed is just one example of a very important principle in the growth and development multicellular organisms. In very small orgainisms such as  Nematodes in the genus Caenorhabditis, one can trace the placing of pretty well every cell (and I take my hat off to the people who actually, incredibly, have done so.) But in those microscopic worms it takes only a few hundred cells to build an entire body. In larger creatures such as Drosophila, Homo, Balaenoptera, and Sequoia, there are so many cells that the information in the genome cannot determine the placement of each one. Instead development is determined only down to the overal pattern, leaving details to the growth process, much as the roots of a plant grow according to a specific pattern, but the details depend on soil conditions, nutrients etc.

Anyone who has much experience of dissection of biological specimens can tell of finding all sorts of minor deviations from the expected patterns. I personally remember for example, a pair of kidneys, one served by the normal renal artery in the form of a short spur off the Aorta, the other renal artery coming from far to the anterior, then travelling straight and in parallel to the aorta until it reached the normal point at which the renal artery would have emerged from the aorta, then turning off towards the kidney. This was in several ways puzzling, but from the practical point of view it probably was the result of some minor embryological accident, though in general obedience to the pattern of growth. 

Now, in the examples you mention, and far more drastically in microscopic patterns such as the formation of fingerprints or retinal blood vessels, the details are far, far finer and the scope for variation is far broader, in that only a general pattern needs be observed to ensure that every bit of functional tissue is properly served by blood vessels, lymph vessels, connective tissue and so on.In fact, throughout life there are adjustments according to life patterns, scars and so on, sometimes the adjustments are barely noticeable, sometimes drastic.

It is all part of the developmental pattern. Think about it next time you loof at the venation of a lettuce leaf before you crunch it, or the roots of a weed that you pull up, or the entrails of the next sardine you eat.

Eggs... Now eggs... Well they don't vary visibly before their development proceeds past the first stages, but a couple of sunny-side up eggs look so much more cheerfully like a pair of clear yellow eyes staring up at you before you eat them, as opposed to the blearily miserable cataract appearance of eggs done "easy over".

Bon appetite!

Jon