When your customers buy once as a guest and the next time with an account on your store, BigCommerce calculates them as two different customers.
Here at RetentionGrid, we focus on unique email addresses; when we see different orders with the same email, we combine them under one customer profile. That means that BigCommerce will show you more customers than your Grid, although they're all here.
In other words: don't worry, your customers are all there. We just arranged them. If you're still uncertain about this, just shoot us an email.