dependency failure w/ yum (and pirut), ok with rpm -i

Hi,
        I'm completely new to RPM packaging, so if the following is
basic stuff just point me to a web page or FAQ.

I've created a number of RPMs packages (on CentOS 5.2). In particular
one for GCC works fine if I install it with the 'rpm -i' command.
When I build a repository (metadata created w/ createrepo) and access it
through yum (or pirut) the process stops complaining about unsatisfiable
dependency constraints. The output of a yum installation is as follow
(pirut agrees on the result):

  [mav@centos-dev rpm_creation]$ sudo yum install polymath-gcc-4.3.3-native-native
  Loading "fastestmirror" plugin
  Loading mirror speeds from cached hostfile
   * Local Polymath Repository: repository
   * base: mirror.trouble-free.net
   * updates: mirror.trouble-free.net
   * addons: yum.singlehop.com
   * extras: mirror.steadfast.net
  Setting up Install Process
  Parsing package install arguments
  Resolving Dependencies
  --> Running transaction check
  ---> Package polymath-gcc-4.3.3-native-native.x86_64 0:4.3.3-1 set to be updated
  --> Processing Dependency: libc.so.6(GLIBC_2.6)(64bit) for package: polymath-gcc-4.3.3-native-native
  --> Processing Dependency: libmpfr.so.1()(64bit) for package: polymath-gcc-4.3.3-native-native
  --> Processing Dependency: libc.so.6(GLIBC_2.6) for package: polymath-gcc-4.3.3-native-native
  --> Running transaction check
  ---> Package polymath-gcc-4.3.3-native-native.x86_64 0:4.3.3-1 set to be updated
  --> Processing Dependency: libmpfr.so.1()(64bit) for package: polymath-gcc-4.3.3-native-native
  --> Processing Dependency: libc.so.6(GLIBC_2.6) for package: polymath-gcc-4.3.3-native-native
  ---> Package polymath-glibc-2.7.x86_64 0:2.7-1 set to be updated
  --> Finished Dependency Resolution
  Error: Missing Dependency: libc.so.6(GLIBC_2.6) is needed by package polymath-gcc-4.3.3-native-native
  Error: Missing Dependency: libmpfr.so.1()(64bit) is needed by package polymath-gcc-4.3.3-native-native

Now those dependencies are in the rpm file, but not explicitely in my
spec file. rpmbuild added them. Just to minimize possibilities of
errors, I created the RPMs on the same CentOS system on which yum is
later run, although in my understanding this shouldn;t be necessary.

Would somebody be so kind to point me in the right direction and help me
understand the difference between yum and rpm processing of
dependencies?

TIA,

        Maurizio