Well thats slightly disingenuois, worth in acting implys experience. Although i understand why youd think we're talking about 'worth'
Matt was worth more because he was more experenced, percieved to be better, he had proof that he was, substancial proof, its called having a CV in acting.
You say you work in HR, if some one does the same job they get paid the same, thats not quite true is it, theres a banding scale in a lot of jobs, nursing, teaching, NQT (newly qualified teachers) get paid less than band 5 teachers. Their doing the same job.
Based on experience and expertise some people do get paid more.
In this case Foy was seen as new, sugnificantly less experienced, the pressure to promote the series was on Smith, indeed i've only heard of the series because of him, hes the one that did the interviews, launched the series. Based largly on the fact he was doctor who. He was used to interviews, he could speak the press lingo, he knew what to say and when to say it, that tesponsiablity fell to him.
Same way as sone one newly qualied and less experienced in most jobs would be paid less as they would be deemed to have less responsablity.
Yes theres a question over series 2, but that depends on when contracts were signed and wether foy could have been rewarded for the achievements of series 1.