meaning - Usage of "certain" in these newspaper articles

Is the usage of "certain" in these newspaper articles correct?

Excerpt from Mutual funds for P1,000 a month (Inquirer.net) (emphasis mine):

"Because remember, you're acquiring shares. So it's good for the
investor ... You can never get the timing right when you invest, but
if you're doing it on a monthly basis, you're indifferent to how the
market moves, because your objective is really to reach a certain
amount in terms of investment over a certain period of time," she
says.

Cognizant of the market's demand for better yield, Pami allows
switching from one fund to another.

"What we're really pushing for is diversification, maybe have a
certain bucket in fixed income, a certain basket in equity-based funds and then a certain portion in the peso and dollar funds,"
Morales says.

The closest definition of "certain" in this context on Oxford Advanced Learner's dictionary seems to be: "used to mention a particular thing, person or group without giving any more details about it or them". I would like to ask if this definition applies to the usage of "certain" in this context. Or is there another correct definition that applies?

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Another example — excerpt from Hazing eyed in death of graduating UP student (Inquirer.net) (emphasis mine):

The village chairman of Talisay, Tiaong, Quezon, Pedro Panopio, who was designated by the family to speak on their behalf showed the Inquirer a statement written by the duty guard of the Veteran’s Memorial Hospital identified as a certain Jonathan Garduce.

He noted in his handwritten statement that Chris was brought to the medical facility at around 1:08 a.m. Monday by a certain “Dr. Francisco Cruz” aboard a white Toyota Innova with license plates ZXB-393, followed by two other vehicles: a Nissan Trooper (WGL-515) and an Isuzu van (XAS-548).

The closest definition of "certain" in this context on Oxford Advanced Learner's dictionary seems to be: "used with a person's name to show that the speaker does not know the person". I understand "know" in that definition to be the fourth definition of "know" in Dictionary.com: "be acquainted with (a thing, place, person, etc.), as by sight, experience, or report: to know the mayor". Does it apply to this or to another correct definition?

It seems that in this context, though, it is used to state that the names are unconfirmed (not from an official source) or just to emphasize the names. The writer of the news article is not expected to know (be acquainted with) the persons mentioned in the article personally, after all.

Answer

In the case of "a certain amount", the writer means that the investor has chosen an amount, but the writer is not saying what the amount is for some reason.

The writer may not to specify the amount for many reasons. He may not know what it is. In this case, each investor will have his own amount, and the writer's point is that each person should be concerned with the amount he has chosen and not someone else's. It might be a violation of someone's privacy to state the amount. (You probably wouldn't want your banker publishing the amount of your account balance just so he could use you as an example.) The writer could make up an example amount, but then readers might be confused into thinking that the particular number chosen is important. Etc.

In the case of "a certain Jonathon Garduce", the writer means that there is a specific, real person by this name, but he cannot identify him beyond giving his name. If you tell a co-worker, "Fred Smith was here to see you," there's a possible implication that you know who Fred Smith is. But if you say, "A certain Fred Smith was here to see you," that means that a person came who said his name was Fred Smith, but you don't know him. It is, I suppose, a very specific idiom.