The author seriously didn't give an explanation? She's quoted the example from an earlier book, "The Craft of Scientific Writing". In that book, the author Michael Alley explains the problem very clearly (page 92): - Although the author intended "as" to mean "because," many readers would interpret "as" to mean "while". Rather than use the word "as" in this sentence, the writer should use the word "because."

"because" is stronger than "as". It makes it very clear that something is happening as a result of something else, like "因为我去商店，所以他们都去学校“。In this case, "as" could also mean "as something happens", like "当我去商店时，他们常常去学校“.

Thats right. In the second example: the addition of the word "because" means that that the lymphocytes is clearly the main cause for the appearance of T cells. In the first example, the lymphocytes may or may not be the cause of the T cell appearance.