Naively, I'd expect all letters to occur equally frequently. Do you know if this is the case?
There are consequences to the proportion of some nucleotides, the most well-known being the GC-content[1], or proportion in a sequence of bases being G or C (G pairs with C and A pairs with T). There are benefits to a higher GC-content, like the sequence being more stable.
From this the obvious question would be why this matters, since you obviously can't replace an A-T pair with a G-C pair just to increase this proportion. Except that there are ways to do it… kind of. If you look at the codon table[2], you'll see that there are multiple ways to represent each protein encoded by a triplet of nucleotides.
I love how the DNA language, much unlike spoke words, is in close relation to physical properties of the world.
There really is some exciting magic in the interception of biology and computation.