Why Your Customer.io Data Doesn't Match PostHog

This is more common than you think

If you're looking at the same event in PostHog and Customer.io and getting different numbers, you're not alone. The discrepancy is almost always one of three things: events firing at different points in the user flow, inconsistent user identification, or data not flowing reliably between systems.

The three most common causes

1. Events fire at different points

PostHog often captures events on the frontend, when the user clicks something. Customer.io may receive the event from your backend, when the action is confirmed server-side. For most actions there's a small time lag. For some, the event never reaches Customer.io if the backend call fails. Result: PostHog shows 1,000 signups, Customer.io shows 940.

2. User identification is inconsistent

Anonymous users in PostHog get a generated ID. When they sign up, PostHog links that anonymous ID to the real user ID. If your Customer.io integration doesn't receive the identify call at the right moment, you end up with ghost profiles: events attached to nobody.

3. Events aren't being sent at all

An engineer implemented the Customer.io tracking in one part of the codebase. A new feature shipped that bypasses that code path. Events from the new flow never reach Customer.io. You find out when a segment that should have 5,000 users only has 3,200.

How to diagnose it

Start with a specific event you can count definitively. Signups are usually the clearest. Compare the count in PostHog, your database, and Customer.io for the same 30-day period. If they don't match, you have a data integrity problem. From there, the investigation is process of elimination.