To add to the above, PayPal IPN transactions typically appear as "web_accept".
In Express Checkout, if there are any line-item details sent to paypal, the type will show as "cart", and no line-item details, then "express-checkout".
Line-item details are sent only if there are no discounts applied and all the math adds up properly according to PayPal's (overly limited and inflexible) formulae.
Thus, if any coupons or discounts apply to the order, it'll come as express-checkout (although other situations may also cause it to appear as "express-checkout", also).
Having only one item vs more-than-one item can also affect it.
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