RecyclerView往上滑动翻页时findFirstVisibleItemPosition() 所获取的值为什么不会改变？

RecyclerView往上滑动翻页的时候，linearLayoutManager.findFirstVisibleItemPosition() 所获取的值为什么不会改变？

往下滑的时候是会改变的，比如从1到9，往上滑就一直停留在9了。（准确的说：它跳到8，然后又马上变成9）

请大家帮我看看：为什么会发生这种情况？代码：

class PdfScrollListener extends RecyclerView.OnScrollListener {
int currentPage = 0;
int lastPage = 0;
PdfActivity pdfActivity;

public PdfScrollListener(PdfActivity pdf) {
pdfActivity = pdf;
}

@Override
public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
Log.e("Position","============："+linearLayoutManager.findFirstVisibleItemPosition());

LinearLayoutManager linearLayoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
if (((LinearLayoutManager) recyclerView.getLayoutManager()).findFirstVisibleItemPosition() == -1) {
currentPage = 0;
} else {
currentPage = ((LinearLayoutManager) recyclerView.getLayoutManager()).findFirstVisibleItemPosition();
}

//再插入之后的两页

if (currentPage > lastPage && currentPage + 2 < pdfActivity.pageCount) {//判断不超过总的页数就提前插入两页
pdfActivity.goDown(currentPage);
recyclerView.getAdapter().notifyItemInserted(currentPage + 2);
pdfActivity.deletePage(currentPage);

}

if (currentPage < lastPage && currentPage - 2 >= 0 && recyclerView.getAdapter().getItemId(currentPage - 2) == recyclerView.NO_ID) {
recyclerView.getAdapter().notifyItemInserted(currentPage - 3);
}
recyclerView.invalidate();
lastPage = currentPage;

}

}