18 seconds on the WU, but 12 minutes on the CSV? That doesn't make any sense. Let's hope it's buffering a full account of the last 12 minutes when it updates the CSV? Why not just update when the data is available from the sensors? I Guess it really does not matter since WD can read the USB port directly. Nice to see new software though. I will definately invest in a USB based display to give it a try. Great job AcuRite!
It's an artifact of the way the internal buffer in the consoles work. The internal buffer aggregates data to 12-minute intervals. I think it can hold up to two weeks of data. If you don't download the buffer at least every 2 weeks, you'll start losing data. Once it fills, it stops recording.
It's just an option for people who want to keep historical data but don't want a PC running all the time. It's not very useful for real-time data acquisition.
What you would want to do is run the console in "streaming" mode (that's set by using USB mode 3 or 4). That will allow you access to the 18-second data, but you'd have to provide a program to build your own CSV file. As you mention, WD probably uses the streaming mode to acquire data. I wouldn't be surprised if WD can build an 18-second CSV for you.
Acurite's use of the term "USB Mode" is kind of confusing. It's not like USB2 or USB3 people are familiar with for high speed data transfer. The mode setting determines if you are streaming (18-second data) and/or using the internal logger (12-minute data)
The attached chart might make things more clear.