change in tense during a sentence

[Musicgold]

Hi,

I read the following text in a legal document. I don’t understand why the author has changed the tense of the sentence from the present perfect continuous to the past in the later half. Also, when should one use such a structure?

Cash-settled convertible bonds have been a popular means of financing for issuers because the issuance of these bonds had a less dilutive effect on earnings per share than did the issuance of other securities.

Thanks,

MG.

[Mister Micawber]

.
It is a little off-putting, isn't it? Evidently, the writer is thinking the continued popularity of the bonds in the first part and of the original effects of the bonds at 'past' issue date in the latter part. (That is simple present perfect though, not continuous aspect).
.
_________________
Native English teacher at Mister Micawber's

[Musicgold]

Thanks.