The linked transactions have a large number of sigops (241). 

Default bitcoin core transaction selection treats sigop's as though they have a larger size than they do. This means that these transactions are treated as though they have a significantly lower fee rate (half in this case)

https://github.com/bitcoin/bitcoin/blob/82ba0f80a0b56bb160c8be1fddd82dbbc4fb3947/src/policy/policy.h#L37

static constexpr unsigned int DEFAULT_BYTES_PER_SIGOP{20}; 

 thank you very much for your reply, but I don't get how I should change that value exactly.
Is there a "one fits all" solution (like 10?) or does it depend on the amount of sigops in a specific transaction?