You look at your Google Ads report.
Campaign A (Mobile): Spent $500. 0 Conversions.
Campaign B (Desktop): Spent $100. 5 Conversions.
The logical move is to turn off Mobile, right? Wrong. If you turn off Mobile, your Desktop conversions will vanish too. This is the "Cross-Device Paradox."
The "Commuter's Journey"
People don't live on one screen. They research on the go, but they buy when they are comfortable.
Why Basic Tracking Fails
Standard analytics uses cookies. Cookies live in the browser. The cookie on your iPhone Chrome is totally different from the cookie on your Windows Laptop Chrome.
To analytics, this one person looks like two different people: One who clicked and left (Mobile), and one who appeared magically and bought (Desktop).
The Solution: Google Signals & User IDs
Google knows who you are. If you are logged into Gmail on your phone AND your laptop, Google can bridge the gap.
By enabling Google Signals in GA4, you allow Google to use its massive database of logged-in users to connect these dots for you.
How to Act on This Data
Once you understand cross-device behavior, your strategy changes:
- Don't Pause Mobile: Even if CPA looks high, check the "Assisted Conversions" report in Google Ads.
- Optimize Mobile for Research: Make your mobile pages easy to read and skim. Don't force a complex form fill on a small screen.
- Remarketing is Key: If they visited on mobile, hit them with a desktop remarketing ad later that evening to remind them to finish the job.
Conclusion
The customer journey is a zig-zag, not a straight line.
Stop judging your ads based on the last second of the game. Look at the whole match. Cross-device tracking gives you the full replay.