Also, was PP the default/first payment option by any chance?
I've done these tests, repeatedly; removing the PayPal payments option consistently leads to a statistically significant increase in cart abandonment. Some customers strongly prefer PayPal (especially on unfamiliar sites) and some don't have a credit card.
If your customers are from Europe, it's not some, but most customers that don't have a credit card. Credit cards are uncommon here (probably hard to imagine if you are from USA).
Online payments are a fragmented mess in Europe. Most countries have their own preferred online payment method (like iDEAL or bancontact), Paypal is basically the only reliable method that works everywhere.
Paypal is because it's safe. I don't give that website I don't know anything, I give paypal a send order. Vastly different; I know paypal, I know they don't screw with the buyer in case an issue happen, and I know they're a bank here and can't drain my funds.
I hate them when I'm the seller, but my personnal experience with them as a buyer makes it clear as day why I need to deal with them.
This is not a great approximation. In the UK you can assume almost every person has a payment card that works on the internet (debit and/or credit). In Poland on the other hand, the number of systems gets silly (https://m.lot.com/us/en/payment-methods-on-lotcom)
WRT. your number of paying systems, note that there's exactly one Polish payment system there - DotPay. Every other system there is to support other countries (not surprising, given that you've picked up the site of the national airline).
No, they are not. With a credit card you can pay by entering the card number, expiry date and CVC. With debit card you can't do that. The payment must go through your bank. It's a completely different payment flow.
It depends on the debit card. I have both Visa Electron and Maestro, and they have 16 digits, CVV number and they behave like a credit card with 3D Secure when doing online purchases, while taking the money from the account right away.
There's also multifunction credit and debit cards (Mastercard and Maestro, or the Visa equivalent). They are branded like credit cards with a tiny Maestro logo on their backside, so I had to explain and ask the cashier in a grocery store in Amsterdam to try it as it would magically work as a debit card.
My debit card can't be used online except via iDeal, and only sites in this country (largely) support iDeal. I can, however, use it via paypal.
Not only do both have completely different payment processing workflows, but there's another key takeaway:
With a credit card the money going out is the issuer's. If you have a compliant or need to handle a charge, it's not your neck on the line.
With debit? Once the cash is pulled it's pulled. Yes they have protections as well, but while you're fighting with your bank to get your money back it's gone.
Until you're big and known enough buyers don't trust you, but they trust PayPal.