I've seen similar statistics before...
There are a couple different caveats to the data (which is why PolitiFact often rates claims like this as "partly true" or "mostly true" and not "completely true"
1) They often set the time period to specifically cover a range of years that makes the point they're trying to convey
For example (let's pick an unrelated topic) Pretend we were talking about food poisoning.
Hypothetical --
For Burger King, every year for the last 10 years, they've had 10 cases of food poisoning (100 cases for the decade)
For McDonalds, for the first eight years of the time period, the had 2 cases per year, but in the past two years, they've had a couple of high profile incidents that resulted 30 cases per year (total of 76 cases for the decade)
So depending on which way you want to spin it, you can make two different claims, and they'd both be accurate
"Based on data from the last decade, the likelihood of getting food poisoning is actually over 20% lower at McDonald's"
and
"Over the past 2 years, you're three times as likely to get food poisoning at McDonald's"
...would both technically be "factually true" statements, but would be employing some creative presentation in order to put a positive or negative spin on McDonald's depending on the motivations of the author.
It's similar to when people are debating extremism/terrorism for the purposes of being able to label one group or another "worse" with regards to it, the choice to specifically include or specifically exclude certain key dates (of specific events) often gives away the motivations of the publisher.
2) A lot of the data they're aggregating is based on arrest and incarceration data, and by the very nature of being undocumented, it's going to make you far less likely to get arrested for certain situations as there's a certain level of anonymity and "flying below the radar" when you're name isn't in the police systems, and you have no registered address or family members living in the area who may be able to tell police where to find you.
For instance, if a citizen assaults someone, or steals from a store, etc... "Oh, that's Bill Miller, he lives over on 9th street" or "That's Steve Jenkins's son", police have addresses and family names they can look up to find the person. Police have options when it comes time to try to make an arrest. (nothing that in many cases, arrests don't happen at the scene of the crime, they often happen days to weeks later)
3) The glaringly obvious one, the violation of immigration law, itself, is a crime. Therefore, for each person who enters the country illegally of their own will, or knowingly overstays their visa, has committed a crime.
