Email from my Google Apps domain gets SPF softfail due to private IP

I am using Google Apps Email for my domain and I have set the SPF record as recommended by google:

v=spf1 include:aspmx.googlemail.com ~all

However, sometimes my email gets a softfail with a private IP being rejected:

Received-SPF: softfail (google.com: domain of transitioning example@example.com does not designate 172.21.105.3 as permitted sender) client-ip=172.21.105.3

As per RFC 1918 (http://tools.ietf.org/html/rfc1918), the 20-bit block (172.16.0.0 -172.31.255.255 (172.16/12 prefix) is reserved for private internets. Furthermore, I do not use any internal IP on my Internet connection from that range of addresses. (I use one from the 192.168 range.) Why is Google rejecting email from within their own servers?

Please, I need to be able to send authorized mail from my domain, so If anyone could help me I would really appreciate it.