There are two different measures in common use: compression rate, and compression ratio. When you see something expressed as a percentage, then it is always a compression rate, not a compression ratio. Compression rates cannot exceed 100. Compression ratios can exceed 100, but are never stated as percentage.
(1 - (size of compressed version) / (size of original)) * 100
For example, a 66.6% compression ratio would indicate that 2/3 of the storage space has been saved and that 1/3 of the original space is still required. A 1000% compression ratio would indicate that the data has been compressed down to negative nine times its original size, which is of course impossible (at most you could compress down to zero times the original size, 100% compression.)
Compression ratio is always given in ratio form, never in percentage form.
(size of original) : (size of compressed version)
reduced down to some "pleasing" rational, typically in "something to 1" form. For example, no-one ever quotes 31917 : 8352 as a compression ratio, but they might quote 375 : 1
If that hypothetical 31917 original were to be compressed down to size 1, that would be a ratio of 31917 : 1, so in ratio form it is possible to exceed 100 or 1000.