The short answer
The one-millimeter size difference is less important than the fixture voltage, bulb voltage, and regional safety certification.
SpecificationE26E27
Nominal diameter26 mm27 mm
Typical marketU.S. / CanadaEurope / 220–240 V regions
Common supply120 V220–240 V
Best practiceUse an E26-rated bulbUse an E27-rated bulb
Before you substitute one for the other
Never assume interchangeability from physical fit alone. Read the fixture label and use a bulb approved for the local supply voltage.