Difference between this two sentences

Your question is deeper than it may seem to many native speakers. It would have been difficult for a non-native speaker to uncover the key concept of tough movement (see brief excerpt below). I therefore answer your question.

The adjective easy applies logically to the estimating of the matrix and not to the matrix.

The author has wrongly applied easy to the matrix and not to the estimation. They have mistakenly used a construction that parallels “It is not easy to be cured of tuberculosis”, “... to be fitted with a false leg”,”... to be rescued from an avalanche”, where the verb creates a reaction in the subject. There is no reaction in a matrix.

Your own version correctly applies the adjective easy to the estimation. It does so by using the grammatical concept of:

Tough movement = In formal syntax, tough movement refers to sentences in which the syntactic subject of the main verb is logically the object of an embedded non-finite verb. The following sentences illustrate tough movement.

(1) This problem is tough to solve.

(2) Chris is easy to please.

Wikipedia

The link contains extensive further discussion of the concept, too lengthy and formal to repeat here.