OpenSSL Base System vs. OpenSSL port

[Bruno Lavoie]

Hello,

i'm running releng_4_9 and i've installed some port (mod_php4, 
postgresql) that uses OpenSSL, but when i do a pkg_tree these packages 
have in their depedency list the openssl that come from ports.
i know that the freebsd base system have openssl,

but why my ports aren't taking the openssl from the base system? (or 
what is the good way?)
what is/are the main differences between the 2 ones?

thanks in advance
Bruno