But then why do the gtx 770m and gtx 770m SLI have different specs (number of shader units, etc.)?
Thats where the benefits comes with sli cards. Shader units become more in sli for shaders can calculate rendering effects on graphics more better and more precisely in less time. Same is the case with textures, mapping and pixels. But hardware status remains unchanged if you are using same specification gpus, like core speed will remain un-changed nor does memory doubles. Only major disadvantage is producing more heat than single gpu for it uses more power in sli.