—There's a ball pen and some pencils.—There are some pencils

意思一样.
is和are的变化只是因为there be句型的就近原则才产生的.
There be句型的就近原则也是主谓一致语法项目的重点考点.There be句型中be动词的单复数形式由后面挨近的一项的单复数决定.
　如：There is a pen,two books and many pencils on the desk.
在本句中尽管有钢笔、书和铅笔,但是由于离be动词最近的一项是单数a pen,因此,be动词用单数is.
又如：有一本书和三只猫.there is a book and three cats.
反过来,有三只猫和一本书.there are three cats and a book