Here's why:
* Gerunds are verb forms ending in "-ing" that function as nouns. They can be the subject, object, or complement of a sentence.
* Present participles are also verb forms ending in "-ing," but they act as adjectives or part of a verb phrase.
In the sentence "He is contemplating a new career," "contemplating" is part of the verb phrase "is contemplating." It's not functioning as a noun, but rather as the present participle of the verb "contemplate."
Here's an example of a gerund:
* "Contemplating a new career is stressful." Here, "contemplating" is the subject of the sentence, functioning as a noun.