Some in this example means about, approximately. It is difficult to explain the fine shade of meaning. There is almost a hint that you know it is ten years or that you are fairly certain it is ten years, but you cannnot be sure without checking. About tenyears indicates that you are definitely not sure of the number of years.
"I've known them for some ten years" – it seems to me that the "some" adds a little light emphasis to the period in question. It houses a slight air of surprise or sudden realization.
Sometimes it's accompanied by a raising of both eyebrows.
Thanks for the excellent explanation given by you. The nuances of difference between using About and Some , have been explained very well. However , i could not understand why it is incorrect to use "Nearly". Please could you give an explanation on this. I felt that using "nearly" was correct because it is commonly used adverb
About 10 means something like 9, 10 or 11 or perhaps 8,9,10,11 or 12 - you are not sure.In Standard English nearly10 means 9 or possibly 8, 7 if you are feeling optimistic - it has to be less than ten. The idea behind nearly is that you have not attained the figure you specify, but are almost there.