But what's not clear is if those payments are always just the payments from the previous half of the year or if they pay out coupons as soon as they come in. I.e. do they pay out everything they get as soon as the next monthly dividend date comes round?
It's not clear because the prospectus does not specify exactly how the dividend is calculated, only that it will distribute the income periodically.
The ETF will make monthly dividend payments that are not necessarily a direct reflection of the interest received in the past month (or any other period). The dividend may be more or less than the actual interest received from the bonds, but the total paid for any given year should reflect the interest paid for the year (but even that is not specified in the prospectus; although any interest "kept" by the fund should be reflected in the overall value).
This fund makes two types of distributions: distributions of "net investment income" (interest) at least once a year, and distribution of "net realizes gains" (increases in bond values) annually.
From the prospectus:
Dividends from net investment income, if any, generally are declared
and paid at least once a year by the Fund. Distributions of net realized securities gains,
if any, generally are declared and paid once a year, but the Trust may make
distributions on a more frequent basis for the Fund. The Trust reserves the right to
declare special distributions if, in its reasonable discretion, such action is necessary or
advisable to preserve its status as a regulated investment company or to avoid
imposition of income or excise taxes on undistributed income or realized gains
Put another way, the dividends paid are not "pass-through" payments of the underlying interest received. It's a periodic (roughly consistent) payment. Just because the ETF receives X in interest in January does not mean that all of that interest will be paid ad the next dividend date. It will pay some percentage of its value (to make dividend yield stable) - it may pay more or less than X.
So no, you can't add up all of the interest payment of the underlying bonds to calculate how much the next dividend will be. It will likely be the same (in percentage terms) that the last dividend was.
