The reason is that these platforms apparently differ in the implementation of the w3-standards since they have not used each others code. We should be happy to get the similarity as it is and yes, these differences are pain in the ass and will be here a while. You can try and replace Hybrid app by a progressive webapp by upgrading to 9.1 when it comes out and see if your problems have magically disappeared with this new technology.
Until then there is no other solution then testing and bugfixing your app to meet both platform’s styles.
Thanks for your cause-explanation.