Looking for an explanation of this irregularity, I found the following:
Indeed, as with "-ible" and "-able," this is essentially a question of "Well, you should know how the original Latin verbs are categorized and then be aware of the exceptions." This isn't terribly helpful.
The suffixes are usually applied to verbs that have survived the journey from Latin through Old French, Norman French, and Middle English into Modern English. In addition, there are several words derived directly from Latin which have been recently added to English as scientific and technical terms. Frequently, the verbs themselves didn't survive, but the nouns and adjectives formed from them did.
As some of you may know (and somewhat fewer care), Latin verbs fall into four basic classes describing their conjugation. In one of these classes of verbs (in fact called the "first conjugation"), the infinitive forms end in -are. In another class, infinitives end in -ere. This forms the basis of the suffix rules for most verbs: words derived from first conjugation verbs usually get -ant and -ance, the rest get -ent and -ence. But there are exceptions, even to this!
To add to the confusion, there is a class of words (which we will not list here) which end in -ment.
In the long run, we will have to throw up our arms and proclaim "there are no rules here!"
To complicate matters, sometimes both "-ant" and "-ent" possibilities are acceptable, and others are not:
Indeed, why cannot it be done for English?
in words such as independence (Latin dependere) are due to later qualms by English scholars. But etymological respectability did not always weigh heavily with the public, and so the dictionary makers have had to allow both spellings in some common words, notably dependent, dependant, dependence, dependance. To allow free variation like this in some particular words only serves to confuse spellers still further. These relaxations are not, however, very consistent: *independent, unlike dependant, is not permitted. The speller may feel entitled to ask: if I am allowed in dependent/dependant, why is this choice not allowed in independent, resplendent, and abundant? An out and out free thinker might even wonder: if a Romance language such as French, regulated as it is by a fearsomely conservative Academy of scholars, can fix the spelling in French as , , , , very conveniently but quite unetymologically, why cannot it be done for English?
(taken from A Survey of English Spelling by Edward Carney (1994), p. 422)
(Posted by language fan and friend Silas McCracken.)