Large differences between SEC and SEP indicate too many latent variables are introduced in the model and that data noises is also being modeled.

"Acceptable models should
have lower SEC and SEP, high correlation coefficients
and small differences between SEC and SEP. Large differences
indicate that too many latent variables are introduced
in the model and that data noises is also being
modeled."
page 674 in

Visible/near infrared spectrometric technique for
nondestructive assessment of tomato ‘Heatwave’ (Lycopersicum
esculentum) quality characteristics
by Yongni Shao, Yong He , Antihus H. Go´mez , Annia G. Pereir , Zhengjun Qiu ,
Yun Zhang

Journal of Food Engineering 81 (2007) 672–678