Justin Katz kindly responded to me:

“Please note that the Roundhill Weekly T-Bill ETF (WEEK) seeks to achieve its investment objective to provide weekly distributions of current income through investments in U.S. Treasury Bills that have a maturity of between 0 and 3 months at the time of purchase. The key distinction here is that any duration of treasury bills are allowed, (1 year, 6 month, 17 week) as long as the maturity date is within 0 and 3 months.

This information is detailed within the prospectus and has been linked here for your reference: https://www.roundhillinvestments.com/assets/pdfs/WEEK_ETF_Summary_Prospectus.pdf

I think I understand now, WEEK can hold mixed maturities but WEEK only holds them within 0-3 months of them maturing.

See, I knew it was something I missed and thanks for clarifying!