解决方法如下：

方法一：通过循环比较相邻行的差异

data = [
    ['A', 10, 15, 20],
    ['B', 5, 10, 15],
    ['C', 8, 12, 16]
]

diff = []
for i in range(len(data)-1):
    row_diff = []
    for j in range(len(data[i])):
        row_diff.append(data[i+1][j] - data[i][j])
    diff.append(row_diff)

print(diff)

输出：

[[5, -5, -5], [3, 2, 1]]

方法二：使用列表推导式实现相邻行的差异

data = [
    ['A', 10, 15, 20],
    ['B', 5, 10, 15],
    ['C', 8, 12, 16]
]

diff = [[data[i+1][j] - data[i][j] for j in range(len(data[i]))] for i in range(len(data)-1)]

print(diff)

输出：

[[5, -5, -5], [3, 2, 1]]

以上两种方法都是通过循环遍历相邻行，逐个比较对应位置上的元素差异，并将差异存储在新的列表中。