The source for this number is likely thereligionofpeace.com, not a scholarly site. Far-right websites such as Breitbart are using it as a source for similar claims (the number is changing daily, and the 27000 figure is a bit older).
On their website, they state:
[...] we are not making the claim that this is a scientific product.
They also state:
We only include incidents of deadly violence that are reasonably determined to have been committed out of religious duty - as interpreted by the perpetrator. Islam needs to be a motive, but it need not be the only factor.
We acknowledge that a handful of incidents on our list may not fit the traditional definition of 'terror attack.' A small portion, for example, are of honor killings
We usually don't include incidents related to combat, such as in Iraq and Afghanistan, unless it involves particularly heinous terror tactics, such as suicide bombings or attacks on troops sleeping in their barracks or providing medical care to the local population.
We also hope that this list offers moral perspective against so-called "Islamophobia" and other complaints from Muslim identity groups that are petty by comparison.
Their list contains a vague description of each event, but no sources. They say that they provide sources upon request.
This approach - saying they have sources, but not adding them - seems dubious at best. Adding to that that the definition of terrorism is quite broad, and that the source itself states that it is not scientific, but politically motivated, these numbers do not seem trustworthy.
The above should show that the 27000 figure is dubious. I am not aware of any scientific study, government report, or other legitimate source that released proper numbers on the topic. The GTD would be a credible source. They have recorded 62357 incidents in the timeframe (excluding ambiguous cases and unsuccessful attacks). They do not allow filtering by religious motivation, but taking the top 7 (selected by me) islamic terrorist groups - Al-Shabaab, Tehrik-e Taliban Pakistan, Hezbollah, Boko Haram, Taliban, Al-Qaeda, ISI / ISIL, Hamas - , there were 10708 incidents in the timeframe. In comparison, the top 5 (again selected by me) non-islamic terrorist groups - Kurdistan Workers' Party (PKK), Basque Fatherland and Freedom (ETA), Lord's Resistance Army (LRA), Revolutionary Armed Forces of Colombia (FARC), Liberation Tigers of Tamil Eelam (LTTE) - were responsible for 2122 incidents (some may disagree with the classification as terrorist group for some of these groups, but the GTD at least uses well-defined criteria for their selection). This doesn't provide a full answer to the question, but should at least show a general scale.