adjective/adverb

I am not sure what exactly you are asking. The "reason" for one sentence having an adjective and the other having an adverb is probably that they are intended to be example sentences showing the use of an adjective or an adverb to say the same thing.

First sentence: The fact is easy ....refers to the fact, which is a substantiv
Second sentence : The fact is easily described .refers to described which is a verb

1, The fact is easy to describe. = fact / is / easy

2, The fact is easily described. = fact / is described

In the first sentence the infinitive verb, "to describe" is acting as an noun, i.e. verbal noun, so it's modifier is an adjective.

In the second sentence "described" is a verb, so it's modifier is an adverb.